Time Machine cannot complete backup to read-only disk

There is no reason we cannot use flash drives for TM backups, but it has been my experience they are not reliable and I gave up on them for that purpose.

Why. All flash memory is subject to write / erase cycle limits, so they will all eventually fail for that reason alone. Of course solid state drives also use flash memory, but they also incorporate garbage collection and wear leveling algorithms designed to maximize their lifespan. Those proprietary techniques vary by manufacturer, so their quality varies from highly reliable to junk, with commensurate variations in cost.

But those are solid state drives, not cheap flash drives or SD cards, which I have tried and failed to use successfully with Time Machine. TM's repeated writing / erasing potentially millions of tiny files with each backup tends to expose a storage device's shortcomings. SSDs on the other hand tend to fail catastrophically, with no warning.

Hard disk drives fail also, but they often fail with plenty of warning. Typically they become slow as repeated efforts to write to bad sectors fail, and then those sectors are "mapped out" as unusable.

The bottom line is 1: don't expect USB flash drives or SD cards to be reliable with Time Machine, and 2: don't rely upon one and only one backup device, since any device can fail at any time. A robust backup strategy demands three or more. Why? When one fails, throw it out. That leaves you with only two, at least temporarily. "Two is one and one is none" is a good principle to keep in mind with backups.

"Files can't be copied on the backup disc because it is read only"

If TM determines a backup device is unreliable it effectively locks the device from future alteration, to preclude alteration and probable loss of your one and only one reliable, fully restorable backup. It's normal for TM backup drives to be "read only" for that reason.

"Read only" status is the new normal for a Time Machine backup drive that is formatted APFS. My own TM drives (SSD) are the same and the backup process runs just fine.

I can offer no explanation for the failure of your backup process or why the error message would flag that 'read only' status as the issue other than perhaps there has been some directory or permissions issue with your drive.

I do suggest you get your hands on another proper external backup drive, one that is at least 2x the capacity of your startup drive. Format it properly and give TM a try at backing up to that.

Would also suggest that you erase and reformat this flash drive and let TM have another crack at it. But I wouldn't do that until I had another completed backup on hand, hence another ext drive.

Are we to understand you are backing up to a flash drive?

It's completely up to you because everyone's backup needs are different.

"Good backups" must not be confused with an inability to restore them under dire circumstances in which they are required. Of course any backup is better than none, but it is my opinion if a backup is unreliable it's worse than none... and it has been my experience that flash drives were unreliable.