It looks like Visual Studio is throwing the DEP0700 error because your WinUI 3 Optional package isn't recognized as part of a RelatedSet when deploying. This happens when the optional package isn't properly linked to the parent package in the MSIX Bundle.

Possible Fixes:

1. Ensure the Optional Package is in the Same MSIX Bundle

   • The optional package must be bundled with the main app to be considered part of the RelatedSet.

2. Check Your AppxManifest File

   • Verify that the Main Package and Optional Package have matching Package Family Names and Publisher IDs.

3. Use Dynamic Dependencies Instead

   • If your goal is to load extensions dynamically, consider using Dynamic Dependencies, which allow runtime package loading without requiring a RelatedSet.

4. Verify Deployment Order

• Deploy the Main App first, then the Optional Package.

If you're still stuck, you can check out this Stack Overflow discussion or Microsoft's GitHub thread for more details.

if you need any more help i'm her [email protected]