We are using PowerBI online. we purchased pro license.
I checked for my login and I am in Global Administrator role.
Still I found Refresh schedule on Dataset option disabled for me?
It is disabled, because your dataset is not refreshable. You should check What's supported? section of Configuring scheduled refresh article. You didn't mention what is the source of your dataset. In general, Power BI service needs access to the data source to be able to refresh the dataset. This means that either your data source should be somewhere in the cloud (e.g. Azure SQL Database, file on OneDrive and SharePoint Online, etc.), or there is a data gateway installed and configured.