Hi there!
The error appears under not-so-clear circumstances. I tried to replicate it after I noticed it happen when first selecting the character to move and then another area that wasn't green. Also, I had this error while selecting with right-click an enemy before selecting a character to attack with:
action number 1
of Step Event0
for object obj_control:
Pop :: Execution Error - Variable set failed HP - read only variable?
at gml_Script_scr_action_attack
gml_Script_scr_action_attack (line -1)
gml_Script_scr_player_turn
gml_Object_obj_control_Step_0
Hope it can help a little more with the debugging. I can relate to the pain of having errors that are hard to replicate.
Good luck!