unable to sign a PDF document with national certification authority key In Acrobat XI Pro

Hi Omar272341907l9z,

Thank you for reaching out, an sorry about the delay in response.

As you are using Acrobat XI, we would like to inform you that it is an old and unsupported version.

You may or may not experience issues while using this application on the latest OS versions. 

Refer to the steps suggested in the following help document on a similar discussion: https://helpx.adobe.com/acrobat/kb/error-creation-signature-could-completed.html. Check if that helps.

Please try to install Acrobat Reader's latest version and then try to add a Digital signature via Reader.

Check if you still get the error message.

Let us know how it goes.

Thanks,

Meenakshi