From the Forbiddance spell description:
For the duration, creatures can’t teleport into the area
A character may not know the area is warded by Forbiddance, attempt to cast Teleport to enter it, and fail. If they do, does that cost a spell slot?
When a character casts a spell, he or she expends a slot of that spell's level or higher
So my question is: if creatures can't teleport into the area, can they even cast Teleport targeting the area?
As a DM, if a player says "I cast Teleport to enter the area", do I say "you can't cast Teleport to enter that area", or do I say "you cast Teleport, but nothing happens"?
Best Answer
The spell slot is used
There is no RAW statement about failure when casting spells except under spells that take a longer casting times:
However, teleport is an instantaneous spell and your concentration is not broken--you are casting the whole spell.
So we look at two other spells that can block magic/travel: Globe of Invulnerability and Antimagic Field.
Globe of Invulnerability states:
So one spell is preventing the effects of another. It is implied that the spell is cast, and that the slot used, but it has no effect.
A closer spell is Antimagic Field where it calls out teleportation:
Much closer to what is happening in the Forbidden spell. It suppresses teleportation magic in, but doesn't stop the caster from trying and expending a spell slot.
And as AndrewK mentions is his answer; this is only relevant if the teleport is "On Target". Per the Teleport spell, it's entirely possible that the casting doesn't even resolve within the area of the Forbiddance spell. So first the spell is cast using a spell slot, then it is determined if the caster ends up in the Forbidden Zone or somewhere else, and then finally the spell is rebuked with no effect.