After you set up two-step authentication, to log in to your Shopify admin, you must use the authentication method in addition to your email and password. If you've set up backup authentication methods, then you can choose which method to use when you log in.
Steps:
From your Shopify admin login page, enter your email address and your password.
Click Log in.
Optional: If you set up more than one authentication method, then click the method that you want to use.
Authenticate using the method that you set up to perform two-step authentication.
Authenticator app: Open the app on your mobile device to check your code, and then enter the code into the login page.
SMS authentication: Check your mobile device for an SMS message with the code, and then enter the code into the login page.
Security key: Use the appropriate key or device to authenticate.