how do I add a signature to fill and sign

Where are you using Fill & Sign?  It is a website in the Document Cloud https://cloud.acrobat.com/fillsign, a tool in Acrobat/Reader DC, and an iOS app - which are you using?

Are you trying to create a new signature, or to sign a PDF?  What happens when you try to create a new signature?  A screen shot and detailed description would help.

Thanks,

Josh

PS - Tutorial which is for the web app in the Document Cloud, but most of it also applies to the Acrobat/Reader and iOS versions: Adobe Fill & Sign