I'm trying to install sitecore using the Sitecore Instance Manager and on the step where I need to choose network service folder I'm getting this error.
I've provided full access permissions to this folder to everyone but it wasn't help.
Do anybody have any idea how to solve it?
It is known issue. Here is link to workaround.
You should change account that run SQL server instance and grant that account full rights to folder where you will install your instances.