Hope you are doing well and sorry for the trouble.
Are you experiencing any trouble while signing the PDFs or getting any error message? If yes, please share the screenshot of the same for a better understanding.
What is the version of the aAcrobat Pro youare using? To check the version go to Help > About Acrobat. Also make sure you have the application updated to version 21.05.20048 Go to Help > Check for Updates and reboot the compter once.
You would be able to sign the PDFs unless the document is not locked for editing or has some kind of restrictions added to it. For more information about how to sign the PDF using Acrobat Pro DC for students and teachers please checkout the tutorial here - https://www.youtube.com/watch?v=Ytv_zt3INus