Adobe is a company, not a product. Which product do you use: Acrobat, Reader? Which exact version (Including minor)? Where did you get the PDF you want to sign from? Is it certified (before you try to sign)? Is it encrypted (File->Properties->Security)?
It looks like the PDF you are trying to sign has some protections that disallow signing. These protections may come either from certification signature or from encryption permissions or from XFA (it looks like you have an XFA document). If the document author set permissions on this PDF that disallows signing then your only recourse is to contact document author and request a copy of this PDF that allows you to digitally sign.
Thank you for the response. Yes, I was using Adobe Acrobat Reader XI pro. The document I needed to sign is a template that gets completed with information and was designed forsignatures and I have signed it many times before. Secondly, I have saved it many times before and never had these issues.