How to fix “Reboot and Select proper Boot device”?

Hello, I’m trying to install Linux Mint on an old PC (Pentium E5800, 4 GB RAM) that can no longer be upgraded to Windows 11, but it doesn’t work; I keep getting this message when (re)booting the computer:

Reboot and Select proper Boot device or Insert Boot Media in selected Boot device and press a key

Here is what I did:

1. Download the ISO of the latest Linux Mint Cinnamon version and create a bootable USB with Rufus (MBR)
2. Insert the USB and select it in the boot menu
3. Start and install Linux Mint, and select Erase disk (to remove Windows 10)
4. Reboot after the installation is completed

There are no options for UEFI/Legacy, Secure Boot or SATA mode in the BIOS settings.
The message still appears after adjusting the boot order or selecting the hard drive (250 GB) directly in the boot menu.
I also tried grub-install and grub-update in case it wasn’t installed properly.

Can anyone help me solve this issue?