Thanks for reaching here! If you are getting 404 error it means that your web browser was able to connect to the website server or host successfully. But, the requested resource, such as a filename or a specific URL, could not be found.
Could you please check if asset exists on your server, may be your configuration file is having issue. To check this, open your nginx.conf file and check if your site is utilizing the correct root folder path.
Also suggest you check the path if it is incorrect. Also, see your configuration file's rewrite rules to make sure they aren't misconfigured.
Refer to below document link might be helpful:
https://www.azurephp.dev/2021/09/php-8-on-azure-app-service/
https://www.schakko.de/2021/09/08/deploying-php-8-0-applications-with-azure-app-service/
Let us know.