Doc says dig sig allowed, but won't let me dig sign

Hi, I have a fillable PDF that I need to digitally sign, and it won't let me do so. I've checked the extensions and it says that I should be able to digitallyl sign the document. I've filled in the various text areas that I need to fill, but when I get to the area where I need to digitally sign and I open the certificates tool, the options are greyed out and I literally cannot even select digital signature to either insert it in the fillable area OR draw a box for the dig sig to go in. There are no error messages; it just doesn't even give me the option. What's the deal?