Try open your project .csproj that cause that warning, add below Code to manually add the reference:
Note:
- Make sure the path is correct.
- Change every
TheReferenceProject below into the actual project name that you wanna add for reference.
<ItemGroup>
<Reference Include="TheReferenceProject">
<HintPath>$(SolutionDir)TheReferenceProject\bin\$(Configuration)\TheReferenceProject.dll</HintPath>
</Reference>
</ItemGroup>