Can't sign my form - "The FieldMDP expression for this signature is invalid"

I am trying to sign a form and getting the above error. My team has roughly 20 members that each need to sign a copy of this form, and everyone is getting the same error. Is there a workaround or a plug-in that we need to download? Can we use a regular signature instead of a CAC (military Smart Card ID) signature?

Thanks.