/usr/local/share/dotnet/sdk/3.1.100/NuGet.targets(268,5): error MSB3202: The project file "/Users/travis/build/Sihle97/MyStuff1/Problems/Problems.csproj" was not found. [/Users/travis/build/Sihle97/MyStuff1/MyStuff.sln]
Your repository’s Problems directory is indeed empty. It’s a submodule, but it doesn’t seem to go anywhere. You’ll have to fix that.