Hi,
Strange.
The error mentioned "it may be a temporary issue, try again".
Did you try again? Did the error come up once more? Even after restarting Disk Utility?
If so then I probably would first try to reboot and go through the steps again.
It _should_ just work. If not then I suppose we can try from the command line, but using Disk Utility is certainly easiest.
FWIW, this should work on macOS even on the disk that you are running macOS from. macOS has supported live repartitioning of its boot volume for many years and I've used that feature many times too.
--
Wil
