I want to make a new account as I deleted my old one after my holiday. It won’t let me use my phone number
To re-open a closed account you need to email them directly:
If you just deleted the app on your phone, then you account may still be there. Have you tried logging in using the email you registered with?
If you properly closed your account, as Monzo is a full UK bank account, Monzo needs to keep records for 6 years for financial legislation, fraud and money laundering regulations. Monzo also only allow you to have one account. therefore your mobile number is kept attached to your closed account (for 6 years) so that people cannot just randomly close an account and open a second. As @vitaminj states, in the quoted post by @simonb, you have to wait at least 30 days since you closed your account and then email firstname.lastname@example.org, requesting your account be reopened.
Some Monzo staff are on furlough so timescales could vary.