Can't sign pdf despite the document having signing permissions

[default2bpan1tzekcm]

I have recieved a standard form to fill in and sign digitally and I am trying to do it in Adobe Acrobat Reader. All fields can be filled in and I can check all necessary boxes, but when I get to the bottom of the document I can not sign it. The small sign button is grey and unaccessable (i.e. it doesn't do anything when i click on it). If I check the document properties they say that I have permission to sign. I have looked through previous posts and tried some things, but to no succes. If anyone could give any suggestions on fixes or just where to look I would be very grateful. 

[Bernd Alheit]

Can you post a screenshot?

[default2bpan1tzekcm]

I got around the problem by printing the document and filling it in by hand, but thank you anyway!