Can't E sign on Mac

I have a document that i am viewing on on acrobat reader dc on Mac. the document extension states that the document can be digitally signed, but when i click on the sign document button, it is greyed out and won't allow me to sign. a pop up says the author of the form has specified the fields which i can fill. the rest of the form functions as normal and i am able to text, but not add a signature. is there a fix?