Datamart was not found error between two tenants

Hi ilhan_berberler 

Ensure that the datamart you're trying to access is correctly configured to be accessible from your global tenant. This includes verifying any cross-tenant data sharing settings and ensuring that your global tenant has the necessary permissions to access the datamart located in your local tenant.

Access shared semantic models in Power BI as a guest user from an external organization (preview) - Power BI | Microsoft Learn
Since the issue started occurring after a specific date, it's possible that there might have been changes to the credentials or the SQL connection string information. Double-check that the credentials used for the datamart in the global tenant are still valid and have not expired or been changed. Additionally, ensure that the connection string used in the report is correctly pointing to the datamart.
It's also important to review the tenant settings in both your local and global tenants to ensure that there are no restrictions or configurations that might be preventing the datamart from being found. This includes checking for any changes made around the date the issue started occurring.

Tenant settings index - Microsoft Fabric | Microsoft Learn

Best Regards

Zhengdong Xu
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.