This fix has now been uploaded: https://shaunjs.itch.io/shauns-turn-based-battle-system/devlog/484288/hotfix-for... please let me know if you still have issues!
If you want you can quickly implement the fix for yourself if you don't want to replace the project. Change line 120 of oBattle's create event to read as follows:
if (_action.subMenu == -1) array_push(_menuOptions,[_nameAndCount, SelectAction, [_unit, _action], _available]);