Did you get the Kobo app from the Play Store? If so, open the Play Store app on your tablet. Either go to My Apps on the first page and find the Kobo app and uninstall, or search for the Kobo app in the Play Store and select uninstall. Then you can reinstall it, sign into your account and re-sync your books. I hope this helps. I don't have a tablet myself but I do have an android phone and I would think they work roughly the same way.
Maybe someone with an actual tablet will come along and give better advice if this doesn't work for you.
There's also a Kobo forum here at MR with a bunch of very helpful people that may have similar issues:
https://www.mobileread.com/forums/forumdisplay.php?f=223