administrator permissions on a SSAS database
The SSAS permissions are completely independent from SQL Server permissions.

If you don't have permissions, there is one hope:


  By default, members of the local Administrators group are also granted
  administrative rights in Analysis Server. Although the local group is
  not explicitly granted membership in the Analysis Services server
  administrator role, local administrators can create databases, add
  users and permissions, and perform any other task allowed to system
  administrators. The implicit granting of administrator permissions is
  configurable. It is determined by the BuiltinAdminsAreServerAdmins
  server property, which is set to true by default.


Source: https://docs.microsoft.com/en-us/sql/analysis-services/instances/grant-server-admin-rights-to-an-analysis-services-instance?view=sql-analysis-services-2017

I suggest you to read the link above and try to run your Management Studio using an account that is member of Local Administrators group of your SSAS Server