Recently I have encountered this error. Problem is that: user that runs nginx service has no permission to access /var/lib/nginx/tmp file.

Example:

2021/06/02 14:46:27 [crit] 5148#0: *35 open()
"/var/lib/nginx/tmp/proxy/0/01/0000000010" failed (13: Permission denied) while
reading upstream, ...

Solution: Find the user that runs the nginx service(You can find in the nginx.conf file). And add permission to that user for the /var/lib/nginx folder

user yourUserName;
worker_processes auto;
error_log /var/log/nginx/error.log;
pid /run/nginx.pid;
http {
    ...
}

But before doing that we need to globally give permission for SELinux

sudo setenforce 0

Security-Enhanced Linux (SELinux) is a security architecture for linux systems that allows to have more control over who can access the system

sudo chown -Rf yourUserName:yourUserName /var/lib/nginx