SQL on prem mirroring cryptic error

ptmk25 I confirm that mirroring for on-premises SQL server is supported and works in Microsoft Fabric. See more information here Microsoft Fabric Mirrored Databases From SQL Server - Microsoft Fabric

There are few details to check in your situation if the issue you are dealing with is still not resolved.

1. You have to use an On-Premises Data Gateway, aka OPDG, that you need to deploy traditionally on a physical or virtual Windows machine close to your on-premises SQL server. See more information here What is an on-premises data gateway?
2. When you create a mirrored database in Fabric for your on-premises SQL server you have to create a connection first. The credentials you'll use for that connection will need to have Admin permission for your SQL server to initially configre mirroring. Once the mirroring starts working the Admin permission can be removed. See more information here Tutorial: Configure Microsoft Fabric Mirroring from SQL Server - Microsoft Fabric
3. Some steps for mirroring configuration may differ depending on your on-premises SQL Server version. Make sure to choose the correct documentation to verify the prerequisites.

If you find this answer useful or solving your problem please consider giving kudos and/or marking as a solution.