Hope you are doing well and sorry to hear that. As described you are unable to open the PDF file.
Is this a behavior with a particular PDF file or with all the PDFs that you try to open? Please try with a different PDF file and check. If the file is stored on a shared network/drive, please download it to your computer first and then try again.
Also, please share the screenshot of the error message for a better understanding.
Make sure you have the recent version 21.11.20039 installed. Go to Help > Check for updates and reboot the computer once.
You may also try to logout of the application reboot the computer once and relogin with the same Adobe ID and check