boot – Help Installing Ubuntu on iMac 2013 (NVIDIA GTX 775M) in BIOS Legacy Mode

I’m trying to install Ubuntu 20 on an iMac 2013 with an NVIDIA GeForce GTX 775M using the guide from this forum: ubuntuforums.org/showthread.php?t=2209602.

My goal is to install Ubuntu in BIOS Legacy mode to ensure proper NVIDIA driver support.

I have created a bootable USB with Ubuntu 20.
I followed the installation steps from the guide.
However, after installation, when I restart, I get this error:
“No bootable device — insert boot disk and press any key.”
Could you please help me troubleshoot this issue?

Thanks!