Can you send us the outputted code after running the bnsupport tool that Michiel requested? We would like to get more information about your config files and logs to try to help you.
Also, can you give us more information about your server? Have you been able to install any plugin before? Which version of Drupal are you using? Did you modify your instance firewall?
I launched a fresh new Drupal 8.8.4-0 server and I was able to install a theme providing the URL without any issues