App is damaged and can't be opened.
This error appears to be caused by the App Store itself but the exact reason is a bit of a mystery to us, unfortunately. Deleting and reinstalling is usually enough to fix it, but not always. Often the problem eventually goes away on its own with time or after a reboot which seems to imply that somehow it is triggered by an App Store bug, macOS bug, or some other external factor out of our control.
That said, if you are plagued by this error and nothing seems to help, there is one more thing to try which is to delete all of the support and library folders. We have no idea why this would work, but since it sometimes does, it might be worth a try!
Note: This will delete all of the app's configuration data!
1: Quit Twitterrific if it is currently running (although it probably isn't or else you wouldn't be here). You may optionally uninstall or delete it as well.
2: Launch the Terminal app (which is built into macOS - search for it in Finder or with Spotlight if you don't know where it is).
3: We will now run a command which will delete all of Twitterrific's data folders.
WARNING: The following command will delete data! If you modify the command incorrectly, it might delete something you didn't expect! Be very careful here because we cannot help you to undo this and there is nothing we can do to help you recover from it! Proceed at your own risk! There is no undo!
As you will see, the command below has 3 parts separated by a space: The name of the command, options that modify how the command behaves, and a search pattern to locate the target folders.
The command we are going to run is called rm which stands for "remove." This command removes files from your computer. It deletes them completely and immediately and does not put them into the trashcan. Once removed, the files cannot be recovered.
The second part of the command is -rf which tells the rm command to (r)ecursively and (f)orcefully remove the files. This instructs the command to first remove all of the files and subfolders inside of the matched folder and to do so without asking you to confirm them one-by-one.
The third and final part of the command is the pattern that will identify the folders that are to be removed. In this case, we are searching for folders within the normally hidden Library folder for the currently logged in user (presumably you). There are different "Container" subfolders which we need to search within as well as multiple Twitterrific folders that have some additional characters in their names so the pattern includes "wildcard" characters (the * character). This means that the pattern will match arbitrary characters wherever you see a *. The ~ character is another special character which represents the home folder of the current user, which is where macOS stores the Library folder that we are interested in.
Now that you know what each piece of the command does, paste the following line into the Terminal window that you previously opened and press return:
rm -rf ~/Library/*Containers/*Twitterrific*
4: Finally, reinstall Twitterrific (if you uninstalled it) and launch it. With luck, the error will be gone and it will startup normally.