Hope you are doing well and sorry for the trouble. As described, you are unable to sign the PDF file as the areas are all greyed out.
Would you mind sharing the screenshot of the issue for a better understanding?
Is this a behavior with a particular PDF file or with all the PDFs? Please try to sign a different PDF file and check. If its a file specific issue please share the file with us for testing. Upload the file on the document cloud https://documentcloud.adobe.com/link/home/ , generate the link and share that link with us.
Please check that the signing is allowed under document restriction summary. Open the PDF > Go to File > Properties > Under security tab > Document restriction summary.
Also, check the PDF producer of the file. Open the PDF > Go to File > Properties > Description > Check the 'PDF producer' and share the screenshot of the same.
Make sure you have the latest version (20.12.20048) of Acrobat DC. Go to Help > Check for Updates.