It appears to be an issue with the App Store.
Would you please take a look at the following thread in the Apple Support forum and see if you can resolve the issue?
If the problem still persists, please contact Apple Support.
Sorry for the inconvenience.
Melisa H commented
I have attempted multiple times since August 15 to update the Adobe Acrobat Reader app and keep getting the same message (see attached). When I select buy nothing happens. Any assistance would be appreciated.