Thank you for reaching out to the Community! I’m happy to provide some assistance with customer accounts, to ensure this gets resolved. To confirm I’m fully understanding, are you referring to the option; “allow customers to log in from online store and checkout” from within Settings > Checkout and Accounts?
If so, this option will enable a feature at checkout where customers will be able to log into an existing account and/or create one. This will become available during the Information step within the checkout process.
For more information on managing customer accounts, I recommend checking out our Customer Accounts documentation. If you are not seeing the Already have an account? Log in link above the email entry field and have this option enabled, please feel free to Contact Support to allow for further investigation.
Please let me know if you have any other concerns.