(Same things would apply as I lined out earlier. You'd also need to pay attention to the parent directory of that directory (and maybe parents of those). And check if coturn is actually running as that user and/or group.
You might just (for testing) grant all permissions on the files. Put them into a directory that you're 100% sure coturn can access. Like one of Coturn's own directories. And then chmod 666 both files and give them the same user and group as coturn's own config file has. If that doesn't work, it's not the permissions.)
And read the actual error message again. Make sure it says "permission denied". Make sure the filename in the error message is the exact filename and location of your certificate file. To rule out other errors or a typo.
And now that I'm reading your initial post again, it says it can't find them. So maybe it's really not the permissions, but the coturn config doesn't point to the correct location of the certificate files. Usually permissions is a good first guess. But if the error message says sth else, it could very well be what the error message says. Does it go on and tell you where it looked for the certificates?