users unable to get the request access dailog box for requesting the access to dashboard

Hi. The message is not there because they probably already have access to the report. However because the source is Analysis Services they also need a permission for reading the Tabular Model at Analysis Services Server. When working with that source it's not just about power bi permission, it's also about the model permission. Like PowerBi shared datasets demanding the users to at least view the dataset of a different workspace to show the report permission in another workspace. It's the same for AAS. You need to add users to AAS first and then share at power bi. The best approach would be using Azure AD Groups so you can just add people in there instead of opening all AAS configuration. That way is easier and it can be delegated to the owner of the Azure AD Group.

I hope that helps,