Cant access gmail after updating to 18 ios

You could try deleting the email account in Mail and re-enter it or you could go to Gmail on a web browser and get Gmail to issue a passkey which is then written to you keychain and good across devices.

Google 2 factor authentication has several options for authentication; the recommended one is with the Google Authenticator, an app that generates a different random 6 digit code every minute, so if it prompts for an authentication code you must open the Authenticator app, tap on the code (which will copy it), then paste it into the authentication field. But there are others; go to Google’s security help to find others.

I tried this solution and it did not work. It worsened things because now none of my email for this account is on my phone.