I am trying to install an extension and i am getting only this error in the Readiness Check:
allow_url_open is on so I am not sure why this check won't pass.
Can you confirm the same by uploading a php file having phpinfo() call. If you'll run the php file from browser will list the current PHP setting loaded on the server. You can look for allow_url_fopen is enabled or not. You can also find the path of PHP.ini to make sure you are working on the right ini file and restart the server for the changes to reflect.
Run below commands to check:
php -i | grep allow_url_fopen
Or you can follow below links if they work for you:
https://mage2.pro/t/topic/4181/3