This is because of the existed email listener in the current environment (Pega server) is using the email account which is pointing to this email address.
Please check all the listeners in the current environment and check the same email id is being used in the pointed email accounts.
If any existed listener is using this email account which is associated with the email id then disable/remove that email listener and select the email account in the email channel. Then the above error message is not displayed.
Once you configured the email channel configuration, Pega automatically creates the email listener which you can find in the email listeners.
Please make sure that the email ID which you have configured for the emailAccount which is been used in the email listner is not used in other pega instances. If you have configured in the other pega instances at your end there might be chance of getting this error. Please try to delete them as well and check the use-case.