Ubuntu won't boot on old Lenovo

Hi, I'm trying to install Ubuntu on an old Lenovo PC and I can't seem to get it to boot properly after I install it.

I'm using the 24.04.1 ISO on a USB drive, then I go through the "erase and install" process (just using standard options), and once I reboot after the installation I get an error that says "Reboot and select proper boot device or insert boot media in selected boot device and press a key"

Any thoughts on how to get this to boot, or what I might be doing wrong? Might this have to do with some sort of BIOS boot vs. EFI boot? (I've seen these terms trying to troubleshoot this, but I don't really understand what the difference is.). I have checked the BIOS and the main HDD is the first non-removable media in the boot order.

Thanks in advance for your help!