Quick Fix Web developer, Web Programmer, ecommerce, Website checkout

Hello, this is an URGENT job.
I need someone fast and straight to the point.
The website is not working, and i need it fixed pleased.

The web developer that was supposed to do it is not where to be found, and i am in week 3 of not having a fully functioning site.
I can provide in depth “print screen” details if you are interested to fix for me, and futher addional details you may need to get into the “back end”

Checkout.
Essentially customers cannot checkout at all.

There is an error message stating:

“ERROR – PLEASE VERIFY LOGIN CREDENITAL”

When an email has been sent to the customer, and the email has been opened, there is no link/tab/any method for the customer to verify login credentials.

– Customer has no method of confirming & Processing Payment.

I have tampered with the front and backend all weekend trying to resolve the issue myself but could not do so.

Cost to fix is negotiable just let me know