This error is caused by the Data Source connection not having access into the OTLP database. Connect to your SSAS instance and verify the connection string of your data source is accurate. Make sure to update the Security Settings to use the proper Impersonation Info. The impersonation info defines what credentials to use to connect to the data source, independent of who triggers the processing.

Most likely, your Analysis Services is running under 'NT AUTHORITY\NETWORK SERVICE' and your impersonation settings is set to Run As Service Account. In production environments I've worked in, we usually create a windows service account that runs Analysis Services and grant that service account read access to the underlying data sources. In dev environments, I just run the service under my credentials or just set the data source to use my credentials by using Impersonate Account.