There may be multiple “projects” in the folder and VSCode has selected the “wrong” one.
(in VSCode terms “project” means the sln file)
Use ctrl–shift–P and select OmniSharp: Select Project to select the correct project (a .sln file).
If you have the Output window open with “OmniSharp Logs” selected, you will see it reading your csproj’s. Once finished your goto definition will start to work