The spell Forbiddance reads:
Forbiddance seals an area against all planar travel into or within it.
Can this area be mobile? Such as a the deck of a ship?
More specifically, my campaign is in Eberron, can this be used on an airship?
There is no RAW answer
The rules do not address this case (casting an location-based spell on a movable location). You would have to venture outside RAW to find an answer.