sign is not working in the form

I have the attached govermnet form PDF . when i click on sign icon to insert my signuture , i got the next message and nothing happening. 

"the author of this form has specified the fields which you can fill.  Click on any field to stat filling form."

I know the solution is printing the form to another pdf file and add the signature in the new pdf file , but i need to know what is the reason why the sign is not working in this form, is this  bug in the design or security restriction put by the author or what?. If there is any solution to repair this issue in the same form instead of saving another copy of the form ?   Thank you.