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

from the comments by Denis Rubashkin

Try to use "Run as administrator" when running SSMS, it solved the same problem for me

and that worked for me too.

to run ssms as and administrator press the shift and right click on the ssms icon and that will give the options below: