I want to update my page from ossn 5.2 to 5.5 but when I upload the files there are 500 errors on the server.
So I tried to do a subdomain softaculous installation but it still throws an error.
I attach my php settings
It's fine you found a solution for your site, David.
But I can't confirm that Ossn 5.5 would really need PHP 7.4 - it's working flawlessly with 7.2 and 7.3, too.
Most likely your provider's PHP 7.3 environment is missing one or more Ossn pre-requisites. (The same problem you already had here: https://www.opensource-socialnetwork.org/discussion/view/3623/php-73-compatibility-problem-php-71-out-good)
But without an error_log it's hopeless to find the true reason for your issue.
Case A (and use the ossninfo.php) tool from the instruction. It will be found in System Info component