Hope you are doing well and sorry for the trouble. As described you are unable to fill lout the PDF document.
What happens when you try to fill the PDF, Do you get any error message? If yes, please share the screenshot of the same for a better understanding.
What is the workflow/steps you are doing? Are you using the Fill and sign option or are you trying to edit the PDF form?
Is this a behavior with a particular PDF file or with all the PDFs? Please try with a different PDF file and check. Also make sure the app is up to date and try updating it via Play Store (Android) or App Store (iOS) and reboot the device once and check.
Note: Acrobat Reader Mobile is a freeware application with limited functions, unless you have premium services enabled.