If it’s asking for a phone number then it’s trying to create a new account for you.
The normal cause of this is entry of an email address that isn’t associated with an existing account.
Do you, perhaps, have multiple email addresses and entered the wrong one?
Welcome to the Community @Stephanie7!
@Feathers has provided what is likely to be the right answer here. If you’re still having issues, you’ll be able to reach out to our support team by emailing help@monzo.com.
Feel free to get back in touch if you’re still having issues.
Don’t know what you’re talking about 
No this is not the case I have one email and I was making an account I just didn’t continue and I left the app now I wanna continue creating the account and it’s asking for a phone number but it’s saying that it’s already in use
Then you’ll need to call or email Monzo to get some help. They don’t offer support through this community.
Bear in mind email support it slow. So include lots of detail in your original email to avoid back and forth and delay.
I had the same issue, as others have said help@monzo.com or call them on 020 3872 0620 as it’s the only way to resolve this one.
In my case, a quick phone call let them reach out in the app chat (which I could get to if they sent me a message) and it was resolved at their end.
Good Luck 