Time Machine could not create a local snapshot to back up from because there was insufficient free space on the source volume(s).

You may be faced with having to re-format the External Hard Disk and start Time Machine from the very beginning. Please try to the following with all wired peripherals disconnected including your Time Machine External Hard Disk. If that does not work, you can try a different cable and if that does not work, then you're going to need to begin over.

Please restart the computer in Safe Mode per the directions in the  How to use safe mode on your Mac - Apple Support link. 

Note: If you are using a wireless keyboard please either plug it in using the supplied Lightning cable that shipped with it. If you are using a legacy keyboard or a third-party keyboard that does not have re-charging abilities, please use any USB keyboard. Also note that a Safe Mode restart may require you to hold down the shift key for up to 5 minutes before the computer starts in Safe Mode. You will know it is in Safe Mode when you see “Safe Mode” in the upper right of the display.

After restarting in Safe Mode, then restart normally and test.