Try deleting the cache associated with the App Store.
Quit the App Store if it's open.
Open a Finder window. From the Finder menu bar click Go > Go to Folder
Type or copy paste the following:
Click Go then move the Cache.db file to the Trash.
Relaunch the App Store, sign in and try updating the app.
I thought this was a great suggestion but having tried it by every which way (sigining out of the store then quitting then deleting the cache & restarting my Mac but each time it comes up with the message. Annoyingly Tweetbot is now on the launch page of the App Store & instead of buy it says Update but no Apple is teasing me.
Sadly this did not solve my issue.