I have Oracle ODP.NET (ODAC 19.3.1 x64) installed but the relational data wizard is saying "unable to find the requested .NET Framework Data Provider."

I am using LLBLGen Pro version 5.9. Build 5.9.0

I tried installing the Oracle.ManagedDataAccess.dll into GAC but that didn't change anything. We use the Oracle Nuget packages for running the application so it's quite likely that I don't have something installed correctly with ODP.NET but I don't have any idea where to look.

Since quite some time the Oracle ODAC installer doesn't perform an additional step, which we've documented here: https://www.llblgen.com/Documentation/5.9/Designer/Databases/oracleodpnet.htm#registering-odp.net-in-the-gac-and-machine.config Did you perform those actions?

Yes, of course that was the reason. It's the usual "read the documentation last" approach.