From Archon Arcana - The KeyForge Wiki

If two card effects are simultaneously instructing a player that they “cannot” do something and that they “must” or “may” do the same thing, the “cannot” effect takes precedence.

Example: Anna controls a Pitlord (COTA 093) which reads “While Pitlord is in play you must choose Dis as your active house.” On their next turn Anna’s opponent plays Restringuntus (COTA 094) which reads “Play: Choose a house. Your opponent cannot choose that house as their active house until Restringuntus leaves play.” and chooses Dis for its ability. On Anna’s next turn, she both must and cannot choose Dis, but because cannot takes precedence over must, she only cannot choose Dis and must choose one of her other houses instead.

KeyForge Master Rulebook v17.3 March 2024