The users cannot log in to the Cash App account using Cashtag. Cash App $Cashtag is a username that is your identity on Cash App, not login details. You can receive payments using Cashtag.
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The Cash App users can log in with a user ID or password only. One should have a registered mobile number and an e-mail ID to log in Cash App account.
However, if you are unable to log in Cash App account, contact Cash App support to get help.