SuperTokens is an open source authentication solution offering features like: Different types of login: Email / password, Passwordless (OTP or Magic link based).

SuperTokens.com

Hi Team! Our client's email verification link has expired, and he cannot login either sign-up, is it possible to use  sendVerificationEmail function to send him verification link for a second time? Because it seems that it needs some auth or something then I try to use this function

yes it is possible. You can do that via our user management dashboard

Is it possible to do it programmatically without dashboard, just using sendVerificationEmail function? Because maybe we will have many clients with expired links, so it would be very hard to do it via dashboard for each of them

It is possible to do it programatically as well

you can call the email verification recipe's `createEmailVerificationToken` function and then create a link using that and send then link using the sendEmail function from the email verification recipe as well.