Answer: Use Mac OS Extended.
Mac OS Extended, also known as HFS Plus or HFS+, is the file system that was used on all Macs from 1998 until the last couple of years. APFS is the new file system for OSX and it works well for an SSD drive. But you can’t backup to an APFS drive using Time Machine (even if the external drive is an SSD drive). It sounds a bit backwards I know but that’s the way Apple are doing it at the moment.
There’s a work-around: you can use a sparse image to create a DMG to back up to an APFS SSD drive, (see here). I would not recommend this because the Apple recovery mode will not be able to find your backup.
From Apple Support:
Important: You can back up from an HFS+ or APFS-formatted disk to an HFS+ disk; however, Time Machine can’t back up to an APFS-formatted disk. If you select a back up disk formatted as APFS, Time Machine offers to reformat it as HFS+.
[retrieved Oct 7 2020 from ‘https://support.apple.com/en-au/guide/mac-help/mh15139/mac’]
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