An HTTP 404 File Not Found error indicates that the URL specified as for your script is incorrect. For the pass back to function, 2CO must be able to locate and access the script and post the variables to it.

HTTP errors are not generated specifically by 2Checkout. They are a part of the HTTP protocol and are common amongst all web servers. It is therefore beyond the realm of 2Checkouts support to provide you with specific instructions for making these changes as every server is set up differently and 2CO does not provide server administration services as part of our support plan.

To correct this issue, follow these steps:

1. Login to your account.
2. Under the ‘Helpful Links’ section, click on ‘Settings’ near the ‘Look and
   Feel’ section.
3. Check the URL that is currently entered there and correct any mistakes that
   it may contain.
4. Click ‘Save Changes’.