Adding external libraries to the package

Good afternoon. Please advise on the issue of adding external libraries to the package using clio:

In the .csproj file of the package I added the library:

$(ConfLibPath)/EUSignCP.dll

false

I have already changed the HintPath to $(RelativePkgFolderPath)/Assemblies/EUSignCP/EUSignCP.dll,
still, when adding EUSignCP.dll, the system throws an error "File "EUSignCP.dll" is not a valid .Net assembly".
Please advise, what am I doing wrong?