Fill and Sign Greyed Out

I've searched through the posts for this and none seem to have an answer that I can follow.

I have downloaded a document which requires a signature at the bottom, the file allows me to click in the box and type but the fill and sign yourself in adobe pro is greyed out.

I've checked the security settings of the file and there are no restrictions on signing, and I also can't sign by going to tools>use a certificate> digitally sign. I need to sign these regularly so printing and signing is a huge waste of time.