It’s not a mandatory App for TK assured people.
Either TK App users can wait a bit and follow proceedings at TK App funktioniert nicht mehr and [de.tk.tkapp] TK App claims unexpected issue with Google Play Services · Issue #2617 · microg/GmsCore · GitHub, or they are using the App on the wrong OS (where it can fail anytime by design, like right now).
A workaround is mentioned at both links.