Welcome to the community, but please don’t provide links to client area pages. They reveal your hosting account username, and nobody but you can access them through the link anyway.
Going to your website, however, I can tell by the error message that at least some of your files use capital letters in their filenames.
Can you please check and see if your index file is completely lowercase? Most web hosting companies run Linux on their servers, which includes InfinityFree. Filenames are case-sensitive on Linux. I would highly advise using only lowercase letters in your site’s filenames in the future.
I can see a SQL exception instead, which notes a clear error that is caused by using localhost as the OP’s database hostname, so it’s not a file name problem.
As for that error on the Client Area, there is a new feature which checks for any index file on your domain’s htdocs folder, though it takes a while to update and you can ignore that if you’re sure you uploaded your index file here.
That was a bug in the directory index checker. It’s fixed now, and it correctly shows that you have uploaded an index.php file.
Regardless, it’s completely unrelated to the actual error message on your website though. Which is this error:
Fatal error: Uncaught mysqli_sql_exception: No such file or directory
A “No such file or directory” error during a MySQL connection always means you’re using localhost as the database hostname, instead of the hostname you’re supposed to use. You can find the hostname to use on the MySQL Databases page in the client area and control panel.
You can’t change PHP versions on free hosting nor can we downgrade them, as if you request that change on your hosting account, it will also affect most websites on the same server your website is on, risking vulnerabilities not only on your hosting account, but on others on the same website IP. If you’re running an older version of XenForo and got it from somewhere else in the process, I’m also sorry to tell you that pirated/nulled software is forbidden on free hosting and that, if you pay for software, you also have to pay hosting that suits for it instead of going the free way for software you really like.
Technically it’s possible to have different versions of PHP per account and/or per website. Free hosting used to have it and premium hosting still does. But on free hosting it was removed due to problems with stability and security.
This. Pirated software is illegal and we don’t allow illegal activities. Pirated software also often contains malware you’re installing on your website by using that software.
Plenty of free options exist to build a forum, like MyBB, Vanilla Forums or Flarum, so please consider using those unless you are willing to pay for the official license.