Error NU1105 Unable to find project information - The project file may be invalid or missing targets required for restore

I also got the same after upgrading to version 15.6 of Visual Studio 2017.

Closing VS and deleting the .vs folder fixed it for me.

For me, the casing of the project file on disk did not match the casing in the solution file.

Say I had a solution with LibraryA.csproj and LibraryB.csproj, where LibraryB.csproj has a reference to LibraryA.csproj. Having an incorrect casing for LibraryA.csproj in the solution file would cause NU1105 when building LibraryB.csproj:

Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "LibraryA", "LibraryA\Librarya.csproj", "{24DEBB3B-762A-491D-8B83-6D078C0B30C0}"

I started seeing this problem after upgrading to version 15.5 of Visual Studio 2017. I did not encounter this problem with version 15.4.5.