Hi @oJJ!
Make sure that SQL Server Analysis Service is up and running. You could confirm it from SQL Server Configuration Manager.
Verify that the SQL Browser service is up and running.
Make sure that there is no firewall or antivirus block the connection.
If you finished above troubleshooting steps and still cannot connect to SSAS, please right click properties in SQL Server Configuration Manager and change Built-in account to “Local System”.
For more information, please refer to this blog.
Best regards,
Percy Tang