SSAS Tabular: ImpersonationMode that is not supported for processing operations
I agree the documentation on this is a bit terse, but the error message on the other hand is very clear. ImpersonateCurrentUser is not supported for models attached to a SSAS instance.

This is mentioned in the documentation of the ImpersonationMode Enumeration:


  ImpersonateCurrentUser: Not supported for tabular model databases attached to an Analysis Services instance. 


As well as in the documentation on Impersonation (SSAS Tabular) under the Options header:


  When configuring impersonation, or when editing properties for an
  existing data source connection in Analysis Services, you can specify
  one of the following options:
  
  
  ImpersonateWindowsUserAccount
  ImpersonateServiceAccount