If the browser returns the error 502 Bad Gateway, make sure that the Apache service is running on the server:
# /etc/init.d/httpd start //RedHat based
# /etc/init.d/apache2 start //Debian based
The status of the Apache service can be also checked from the PP Control Panel: Tools & Settings > Services Management
Also you can install license by CLI command:
# /usr/local/psa/admin/bin/keymng --install --source-file <source path>