I'm getting it when I was trying to modify an existing reservation, even when unselecting use original rate plan. But booking from scratch worked.

The Promotion Coupon you've selected - 155341 is not available for this rental either because that location isn't participating or some requirement for the offer hasn't been met. We have removed the Promotion Coupon from your request to provide you with the best rate. There may be other offers in the Special offer section which do apply, so you could check there for an alternative.