The forbiddance spell description states:
You create a ward against magical travel that protects up to 40,000 square feet of floor space to a height of 30 feet above the floor. For the duration, creatures can't teleport into the area or use portals [...] to enter the area. The spell also proofs the area against planar travel, and therefore prevents creatures from accessing the area by way of the Astral Plane, Ethereal Plane, Feywild, Shadowfell, or the plane shift spell.
In addition, the spell damages types of creature that you choose when you cast it. Choose one or more of the following - celestials, elementals, fey, fiends, and undead. When a chosen creature enters the spell's area or starts its turn there, the creature takes 5d10 radiant or necrotic damage (your choice when you cast this spell). [...]
Can you cast the conjure elemental spell to summon an elemental within the area of effect of the forbiddance spell?