System.Resources.MissingManifestResourceException

I converted my WiseJ app from VB to C#, it ran fine, after I tried adding a .NET 5 API project to the solution, the Microsoft Visual Studio Solution File was updated from VisualStudioVersion = 15.0.27130.2027 to VisualStudioVersion = 16.0.31829.152 and now I get this error

Could not find any resources appropriate for the specified culture or the neutral culture.  Make sure “FixityVB.Properties.Resources.resources” was correctly embedded or linked into assembly “FixityVB” at compile time, or that all the satellite assemblies required are loadable and fully signed.

The exception is thrown here:     System.Resources.ManifestBasedResourceGroveler.HandleResourceStreamMissing(string)

Tried cleaning, rebuilding, deleting bin and obj folders, no dice.  Has anyone experienced this, and what is the appropriate fix?

thanks

Andrew