Hope you are doing well and sorry for the trouble. As described, you are unable to type 'A' and when you press SHIFT + "a" you get the letter "n". All other letters work fine.
Is this a behavior with PDF files only or with other files as well like MS Word, Excel? Please try to type the letter 'A' in MS Word or Excel application and check.
What is the workflow/steps you are doing to edit scanned PDF?
Please try to reboot the computer once and see if that helps. Also make sure you have the latest version 21.01.20145 installed for Acrobat DC. Go to Help > Check for Updates and reboot the computer after updating the application and check.