Cannot find source for binding with reference with RibbonComboBox

I'm trying to learn WPF and seem to be getting an error I really don't fully understand.

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='System.Windows.Controls.Ribbon.RibbonWindow', AncestorLevel='1''. BindingExpression:Path=WindowState; DataItem=null; target element is 'Ribbon' (Name=''); target property is 'NoTarget' (type 'Object')
System.Windows.Data Error: 4 : Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='System.Windows.Controls.Ribbon.RibbonWindow', AncestorLevel='1''. BindingExpression:Path=IsActive; DataItem=null; target element is 'Ribbon' (Name=''); target property is 'NoTarget' (type 'Object')

Here a section of my XAML. I can't tell if the errors just started, but I have been tweaking code that works with the two RibbonComboBoxees under the Home header. (The <Ribbon> element is in a <Grid> element.)

Can anyone help me to try and resolve this issue? The code isn't quite behaving correctly and I can't tell if this error is related.

<Ribbon>
    <Ribbon.ApplicationMenu>
        <RibbonApplicationMenu>
            <RibbonApplicationMenuItem Header="_Preview" ImageSource="Images/preview.png"></RibbonApplicationMenuItem>
            <RibbonSeparator></RibbonSeparator>
            <RibbonApplicationMenuItem Name="Save" KeyTip="S" Header="_Save" ImageSource="Images/save.png" ToolTipTitle="Save" ToolTipDescription="Save the current article and keep editing" Click="Save_Click"></RibbonApplicationMenuItem>
            <RibbonSeparator></RibbonSeparator>
            <RibbonApplicationMenuItem Name="SaveAndExit" KeyTip="X" Header="Save and E_xit" ToolTipTitle="Save and Exit" ToolTipDescription="Save the current article and close window" Click="SaveAndExit_Click"></RibbonApplicationMenuItem>
            <RibbonApplicationMenuItem Name="AbortAndExit" Header="_Abort Edit" ToolTipTitle="Abort Edit" ToolTipDescription="Close window without saving changes to this article" Click="AbortAndExit_Click"></RibbonApplicationMenuItem>
        </RibbonApplicationMenu>
    </Ribbon.ApplicationMenu>
    <RibbonTab Header="Home">
        <RibbonGroup Header="Category">
            <RibbonComboBox Label="Category:" SelectionBoxWidth="250">
                <RibbonGallery SelectedItem="{Binding Path=Categories.SelectedItem}">
                    <RibbonGalleryCategory IsSharedColumnSizeScope="True" DisplayMemberPath="Text" ItemsSource="{Binding Path=Categories.Items}">
                    </RibbonGalleryCategory>
                </RibbonGallery>
            </RibbonComboBox>
            <RibbonComboBox Label="Subcategory:" SelectionBoxWidth="250">
                <RibbonGallery SelectedItem="{Binding Path=Subcategories.SelectedItem}">
                    <RibbonGalleryCategory IsSharedColumnSizeScope="True" ColumnsStretchToFill="True" MaxColumnCount="1" DisplayMemberPath="Text" ItemsSource="{Binding Path=Subcategories.Items}">
                    </RibbonGalleryCategory>
                </RibbonGallery>
            </RibbonComboBox>
            <RibbonButton Label="Edit Categories" Command="local:EditCommands.Categories" SmallImageSource="Images\categories_sm.png" ToolTipTitle="Edit Categories" ToolTipDescription="Add, edit or delete categories and subcategories" ToolTipImageSource="Images\categories_sm.png"></RibbonButton>
        </RibbonGroup>
        <RibbonGroup Header="Author">
            <RibbonTwoLineText Text="jwood@softcircuits.com"></RibbonTwoLineText>
            <RibbonButton Label="Set Author" Command="local:EditCommands.Header1" SmallImageSource="Images\h1_sm.png" ToolTipTitle="Header 1" ToolTipDescription="Format selected text as a level 1 header" ToolTipImageSource="Images\h1.png"></RibbonButton>
        </RibbonGroup>
        <RibbonGroup Header="Settings">
            <RibbonCheckBox Name="chkApproved" Label="Approved"></RibbonCheckBox>
            <RibbonCheckBox Name="chkDeprecated" Label="Deprecated"></RibbonCheckBox>
            <RibbonButton Name="btnMetadata" Label="Metadata" SmallImageSource="Images\metadata_sm.png" ToolTipTitle="Metadata" ToolTipDescription="Edit article metadata" ToolTipImageSource="Images\metadata.png" Click="btnMetadata_Click"></RibbonButton>
        </RibbonGroup>
        <RibbonGroup Header="History">
            <RibbonButton Label="Edit Updates" Command="local:EditCommands.EditHistory" SmallImageSource="Images\history_sm.png" LargeImageSource="Images\history.png" ToolTipTitle="Edit Updates" ToolTipDescription="Edit update history for this article"></RibbonButton>
        </RibbonGroup>
    </RibbonTab>

UPDATE:

Based on IlVic's answer, I changed my Window to a RibbonWindow. But then I'm getting the following error.

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='System.Windows.Controls.ItemsControl', AncestorLevel='1''. BindingExpression:Path=HorizontalContentAlignment; DataItem=null; target element is 'RibbonGalleryItem' (Name=''); target property is 'HorizontalContentAlignment' (type 'HorizontalAlignment')

Solution

Your issue is cause by your Ribbon control. If you take a look to its default style (you can use ILSpy), you will find this MultiDataTrigger:

<rd:MultiDataTrigger>
    <rd:MultiDataTrigger.Conditions>
        <rd:Condition Binding="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ribbon:RibbonWindow}, Path=IsActive}" Value="False" />
        <rd:Condition Binding="{Binding Path=(SystemParameters.IsGlassEnabled)}" Value="False" />
        <rd:Condition Binding="{Binding RelativeSource={RelativeSource Self}, Path=IsHostedInRibbonWindow}" Value="True" />
    </rd:MultiDataTrigger.Conditions>
    <rd:Setter TargetName="PART_TitleHost" Value="{windows:DynamicResource {x:Static SystemColors.InactiveCaptionTextBrushKey}}" Property="TextElement.Foreground" />
</rd:MultiDataTrigger>

As you can see its first condition is looking for an ancestor whose type is RibbonWindow. You will easly find another MultiDataTrigger which as a condition that looks for WindowState property of a RibbonWindow control too.

I guess that your XAML is put inside a normal Window. So you need to replace the Window object with a RibbonWindow one, something like:

<rib:RibbonWindow
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:rib="clr-namespace:Microsoft.Windows.Controls.Ribbon;assembly=RibbonControlsLibrary"
        x:Class="[yourWindowClassName]">
    <!-- Put your Ribbon Control somewhere here -->
</rib:RibbonWindow>

I hope it helps.