★Error "U command was disabled. Use UNDO command to enable it" when perform undo command

Issue:

[UNDO] command and the shortcut key [CTRL + Z] cannot perform undo operation, the command bar prompts "U command is disabled. Use UNDO command to enable it.", the Quick Access Toolbar [Undo] button is also in grey (disabled state).

Solution:

Enter the command [UNDO], enter [A] and press ENTER to set the control type as ALL.