/index.php?route=checkout/confirm: Failed to load resource

I will appreciate your help.

This is a fresh installation of opencart 3..0.3.6
I am testing end to end of the solution.
On confirming order - ( with successful payment going through) I discovered the success page is not displayed rather is remained on the checkout page

1. I checked the error log and it is clean. no error recorded.
2. I used F12 to enable developer mode then I saw this error :
/index.php?route=checkout/confirm: 1 Failed to load resource: the server responded with a status of 500 (Internal Server Error)

3. I have checked to ensure under design > layout > checkout that is configured.

Because of this, the successful order is not updated on sales > orders

No extension has been installed yet. It is basic opencart
is there something I have missed out?

/index.php?route=checkout/
I don't really like that forward slash / as shown in your Post,
are you sure, to have it all configured correctly in your config.php file?

2. I used F12 to enable developer mode then I saw this error :
/index.php?route=checkout/confirm: 1 Failed to load resource: the server responded with a status of 500 (Internal Server Error)

Error / server access logs?

Which payment method you are using?

With an Internal Server Error 500 there usually are more details about the error in the server's error log, ask your webhost for the log entries for the exact time when your error occured.