El Capitan install can’t be verified, regardless of date set

I’m trying to install OS X El Capitan on a 2008 MacBook (Mojave is in no way optimized for 2 GB of RAM), but trying to run the installer off of the bootable install USB results in an error message saying that the installer can’t be verified and could have been tampered with.

I’ve tried disconnecting from Ethernet (Wi-Fi isn’t an issue, I don’t have an antenna) and using the terminal to set the date to various different dates between 2015 and 2018, which usually works when I’m trying to install Mavericks, but I still get the error.

Yes, I will upgrade the RAM and hard drive, but I’m prioritizing other repairs/upgrades such as the display, trackpad, and logic board

2008 Unibody MacBook, Mac OS X 10.11.whateverversionapplestillhasthedownloadfor

I’ll probably just go for Mavericks if I can’t get this to work