George-field if you follow any guide, make sure it's for Flarum beta 8+
It's hard to tell what went really wrong, but I see multiple issues in that config file:
The following lines are for beta 7 only, they must be removed:
location /api { try_files $uri $uri/ /api.php?$query_string; }
location /admin { try_files $uri $uri/ /admin.php?$query_string; }
The FPM config is commented out, so PHP most likely doesn't run
The FPM config references PHP 7.0, which is not supported. Make sure you installed PHP 7.1 or later.
The root should include /public at the end.
Maybe there's some file permission issues as well.