Can't backup with Time Machine.
I worked with Apple Support the other day and the technician has me erase system and reinstall macOS. Subsequently it backed up fine. But since then does not.
iMac 21.5″, macOS 10.15
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That seems rather extreme. Did he or she have you follow the steps in If you can't back up or restore your Mac using Time Machine?
The third bullet point recommends using Disk Utility to "repair" the backup disk:
- If your backup disk is connected directly to your Mac, use Disk Utility to repair the disk.
In addition to that, I suggest that you also repair the source disk. Follow those instructions, but where it says "Select your disk in Disk Utility" select the internal disk.
As it states: Repair volumes, then containers, then disks — in that order.