The PDF you are trying to sign might not be Rights extended, Reader is only the read only software and is not meant for editing modifying or signing the document. For making Adobe Reader to perform certain task we need to make PDF Rights Enabled. That can be done via Adobe Acrobat.
I would recommend if you ask you to contact the creater of the PDF and ask him to send the PDF which is Reader Rights Enabled.
How do you make reader rights enabled in Acrobat? I have an editable PDF that will not allow me to sign
craigs8888, do you understand the difference between Acrobat (paid for software) and Acrobat Reader or Adobe Reader (free)? I ask because reader rights are used in Reader but made in Acrobat. Which do you have, and which version of each?