Unable to utilize 'Certificates' for digital signature

I'm using Adobe Acrobat Reader DC v.2019.008.20074 on an Apple MacMini, macOS Sierra v.10.12.6, and I can't utilize 'Certificates' to digitally sign a document.  When I click 'Certificates', I get the following message box, "Cannot Save Form Information.  Please Note:  You cannot save a completed copy of this form on your computer.  If you would like a copy for your records, please fill it in and print it." 

Why am I getting this message?  Why are the 'Certificates' options greyed out and unavailable?

Thanks,

J