Our servers contain a security feature that helps prevent outsiders from using 2FA to confirm if an account exists. Typically, if an outsider wanted to know whether or not a particular email had an account with us, they could try to sign into the account with an invalid password, and if they are shown a 2FA prompt, this means the server is confirming that the account exists. With our security feature, when someone tries to sign in using any email and password, they may receive a "decoy" 2FA prompt, regardless of whether or not the email address they entered is actually a valid account on the server.
If you're encountering the 2FA prompt without having previously enabled 2FA, you may be typing in your account email incorrectly, or your original account email may have a typo in it. Please send us an email at firstname.lastname@example.org, and we will do our best to assist you. Make sure to send your email using your Standard Notes account email to speed up the verification process.