Hi @Susan4927
Is it possible that you've already got an Airbnb account already set up?
This happened to me a while ago, when I tried to make a booking, and it's because I'd forgotten about an old account that I set up years ago but never used. I logged into the old account and deleted it and was then able to update my phone number to my correct account.
If you need more help with this I recommend getting in touch with Airbnb by clicking this link and scrolling to the bottom, where you'll see a black box with a "contact us" button.
Jenny