Greetings, IBRAHIMSHOUK!
Thanks for reaching out to the Apple Support Communities! It sounds like you are getting a message that you iCloud account already exists when you try to sign into Mail on your iPhone and I am happy to help out! The first thing I recommend is going to Settings, then tap your Apple ID at the top. Once there, go to iCloud and check that the Mail is turned on (green).
Let me know the results and have a great day!
Thanks for reaching out to the Apple Support Communities! It sounds like you are getting a message that you iCloud account already exists when you try to sign into Mail on your iPhone and I am happy to help out! The first thing I recommend is going to Settings, then tap your Apple ID at the top. Once there, go to iCloud and check that the Mail is turned on (green).
Let me know the results and have a great day!