Why am I not receiving emails on my iPhone 12 mini?

Probably because your phone is not logged in to your email provider’s server, or the username or password is wrong. Possibly because the email provider’s server is broken.

And usually the easiest way to fix the first cause is to go to Settings/Mail/Accounts, tap on the account and delete it, then restart your phone and go to Settings/Mail/Accounts, tap Add Account and add it back.