Hope you are doing well and sorry for the trouble. As described you are unable to digitally sign the PDF
Do you get any error message when you try to sign the PDF? If yes, please share the screenshot of the same for a better understanding.
What is the version of the Adobe Acrobat DC you are using? To check the version go to Help > About Acrobat and make sure you have the latest version 21.05.20048 installed. Go to Help > Check for Updates and reboot the computer once.
Are you on Mac or Windows OS and what is the version of the OS? Please check for any missing/pending update for your computer and try updating it and see if that works.