The first thing to do is check the file, which means fixing FTP access to you can get access to the file (the file manager also uses FTP). This is the guide to do that:
In your case, logging into the control panel fixed it, which I did to be able to check the file.
Checking the file, I don’t see any obvious issues with it. Nothing that would cause the file to be unparsable.
But I’m not a Prestashop expert, and I didn’t make the Softaculous installation bundle.
If you suspect that Softaculous messed up the file, you could try to download a fresh copy of Prestashop from the website (make sure to use the same version!) and then replace the invalid file in your website with the one from the official download.
If that doesn’t do it, you may want to find a Prestashop expert to help you with this. If you have a (seemingly) correct Prestashop error that gives a vague error about invalid files, that sounds to me more like a Prestashop issue than a hosting issue.