Hello @stevenlrug275e,
This issue occurs when str_replace is being passed a null value in your code. To identify which module is causing this, you can use the following command to search for the str_replace function in your custom module and Theme:
Command:
grep -r "str_replace"
Run this in command prompt in your app/code or app/design directory, and then review the file where it's used. Ensure that you check whether the variable being passed has a value before applying the str_replace function. If necessary, add a validation check for the variable before using str_replace.
If you are in developer mode, you should be able to see the full stack trace on the var/log/debug.log file.
If you don't find it there, you might resolve to go into the vendor/magento/framework/App/ErrorHandler.php and on the beginning of the function (line function 47) add a debug_print_backtrace(); to get the full stack trace on the error. This way you can see where this is been triggered and reach to the correct location to fix.
Make sure you have the error reporting set correctly on your php configuration so you can see the error on the browser.
If you find str_replace function to pass the null data then you can put like this:
str_replace($data ?? ' ']);
this should solve deprecated functionality problem and checkout will run smoothly.
If the issue will be resolved, Click Kudos & Accept as a Solution.
