Can you help me with "returning to a single partition"? How can I do it?I think your error probably is saying that you can't re-join the partitions (removing the boot camp Windows partition, and rejoining that partition to the normal system partition.)
This is usually because you have modified the partitions on the drive. You added the USER DATA partition, so that's the culprit. The modified partition map (with more than just an EFI, boot system, Recovery system) forces you now to use other methods to remove Windows. The modification that you made to the partitions, when you added the data partition, means that you cannot simply use the Boot Camp Assistant to remove the Bootcamp partition. That is because the Boot Camp Assistant doesn't know how to re-join the partitions, when the partition map is not the same as when you first used Boot Camp to make the windows partition.
The easiest way now is to back up your drive (both partitions), then erase, returning to a single partition. Reinstall macOS, and restore your apps and files.
Boot to recovery system.Can you help me with "returning to a single partition"? How can I do it?