Best thing is probably to start over. Follow these steps and try again:
-First, open Disk Utility and do a Disk First Aid on the internal drive to insure it is good shape.
-In SystemPreferences->Time Machine, uncheck "Back Up Automatically".
-Next, press the select disk button, select the current and click remove.
-Close System Preferences and reboot. This is to remove any possible remaining TM data.
-Completely erase the external drive and set up as GUID partition scheme and APFS.
-When this is complete, if prompted to enable using this drive for Time Machine, allow it.
If not prompted, open Time Machine in System Preferences, select the disk, and check backup
automatically.