When backing up a device, iMazing shows you a dialog saying there is not enough free space on disk to backup the device. But iMazing and Finder are not showing the same available space and according to Finder, enough free space should be available.
iMazing shows you the real available space on your local drive but Finder shows you the available free space + purgeable space as shown in the above screenshot. This is due to the fact that Time Machine snapshots on APFS are considered as purgeable but the backup agent will calculate the real available space only and won't be able to ask macOS to purge Time Machine snapshots.
See this thread for more info: https://apple.stackexchange.com/questions/304651/high-sierra-shows-wrong-disk-usage
Unfortunately, the only solution is too free up some space on your hard drive or to turn off Time Machine so Time Machine snapshots are removed.