Are you not able to add signature to the document or not able to Sign In in the Adobe Reader? Looking at the description given above, seems that you are unable to "Sign" a document. If yes, then couple of things that you need to keep in the mind or check-
Does document's properties allows to Sign it? Open PDF>right click "Document Properties">Security>Signing
If working on older version of Adobe Reader then do update it to the latest Adobe Reader DC.