Continuous 500 Error When Upgrading

I've tried upgrading my current Enthusiast (3.1.6) to the newest version multiple times and I consistently get a 500 error whenever I try to load the admin panel. I've backed everything up so nothing has been lost, this is just an issue of frustration, really. I'm not sure what I'm doing wrong.

IE: My fanlistings are on a subdomain "test.blahblah.com" and the enthusiast admin is at "test.blahblah.com/admin/" at the moment. I go through each step for upgrading, check to make sure all information in the config files is correct and upload all the files as needed to make the script run, and when I try to load the "test.blahblah.com/admin/" it gives me a 500 error with a "can't handle this request" statement. It just won't load at all.

1. I'm using the same database information from my old config files in the new config files--both for the admin panel and each individual fanlisting (IDs, database names, etc).
2. I haven't loaded the install.php file since this is an upgrade, not a new install as the instructions stated.
3. I have uploaded everything required in the instructions from only the enthusiast and samplefl folders--I don't use a collective--to overwrite my current files.

I know there's no support past upgrading, but I'm not sure what I'm doing wrong here. I know I have all my database variables right because they match what I currently have and that's working with the older Enthusiast script I'm using now. My PHP version is 7.4 and my SQL version is 5.7 currently. Any help or ideas for how to troubleshoot this would be appreciated.

*I accidentally checked the confidential box thinking it was a standard "I've read and agreed to" statement. I had to repost this issue without confidentiality so more people can see it. My apologies.

Edit: After adding the error check code to my test.blahblah.com/admin/index.php file, this is the error message I receive:

Fatal error: Default value for parameters with a class type hint can only be NULL in /public_html/test/admin/mod_settings.php on line 33

Here is line 33 in mod_settings.php: function get_setting($setting, bool $reportNotFound = true)

Any ideas?