To be clear, you can’t use chmod on free hosting. But all files and directories have the 755 permissions which should be sufficient for any scripts to work, although don’t will ask for 775
It looks to me like this is a coding error in header.php, at least according to the error message on your site. Is this an off-the-shelf script you got from somewhere, or did you make it yourself?
I had a quick look at the software you are using. You seem to be trying to install something called eFiction. The issue is that this particular piece of software hasn’t been updated since 2010. The PHP programming language has changed a lot in the 16 years since then, and it’s very likely that eFiction doesn’t work on modern PHP versions without substantial modifications.
Unless you feel up for that yourself, I suggest to look for alternative software for your website that is still actively being maintained.