Hello ruby_king2006,
Which device are you using the Mail app with? Just to clarify, it seems like this is happening with an iCloud Mail account, is that correct?
If you're using iCloud Mail, this link outlines the steps to take to set up iCloud Mail on your devices: Set up iCloud for Mail on all your devices
Just to clarify, do any errors or messages appear when you try to sign in? If you're not able to sign into your Apple ID/iCloud account, this link has some additional steps that may help: If you can't connect or sign in to iCloud
If you need additional assistance after reviewing the information in the above link, please check out the "If you still need help" section of that same page.
If you still need help
If you still need help signing in to iCloud, update your iPhone, iPad, and iPod touch to the latest version of iOS or iPadOS, and your Mac to the latest version of macOS. See the iCloud system requirements for more information.
Thanks!