Time Machine under the latest Mojave

I used Disk Utility to 'erase' an external drive to APFS and tried to use it as a backup drive for Time Machine.

Time Machine says "Oh no! You can't use that! You must have it formatted as Mac OS Extended (Journaled)".

So I went back to Disk Utility and when trying to 'erase' it back to 'Mac OS Extended (Journaled)' that option no longer appears in the 'Format' dropdown.

What can I do? Is there some sort of Terminal command I can use to get that external drive back which I especially bought for Time Machine?

Thanks in advance.