When you gain the third cantrip spell for Warlock at level 4, you are unable to place it on the hotbar to use. I attempted Mage Hand, Minor Illusion and one other I cannot recall presently. I can only assume it is only that third cantrip as upon changing the feat to Wizard Initiate and picking two cantrips there, they could be placed on the hotbar and used just fine while the third cantrip was unable to still.