Page not found error for return URL

Hi,

I don't see any "page not found" error on that URL.
What I see is the error "500 - Whoops, looks like something went wrong."
This indicates that there is a PHP error during the processing of the page.
In order to know where the problem is, you need to:
- turn on the "debug" setting of the Joomla configuration
- rename the file templates/YOUR_TEMPLATE/error.php to something else via FTP in order to use the default error page of Joomla, and not your template's

However, looking at github.com/techjoomla/joomla-payments/tree/master/code/plugins I can see that the plugins there were not updated for several years.
You said you're using Joomla 4.3, but Joomla 4 didn't exist the last time the CCAvenue plugin was updated. So it's likely that there is one or several problems in the code of the CCAvenue plugin to make it compatible with Joomla 4. The error message will help understand what the first problem is.