Can I cast Quarantine Field with Brisela in play for my opponent?
The Gatherer page for Brisela has this ruling:


  For spells with {X} in their mana costs, use the value chosen for X to determine if the spell’s converted mana cost is 3 or less. For example, your opponent could cast Burn from Within (a spell with mana cost {X}{R}) with X equal to 3, but not with X equal to 2.


You should have been able to cast the spell, so you should report this as a bug.