See https://github.com/notepad-plus-plus/notepad-plus-plus/pull/2062 and https://github.com/notepad-plus-plus/notepad-plus-plus/pull/2336.
Do you see a messagebox with:
Authenticode check failed: signature or signing certificate are not recognized?
Otherwise the most probably issue is that the SciLexer.dll is not placed beside the notepad++.exe.
Do you really want to build and debug scilexer.dll?