I am at my wits end and I am hoping someone sees this and is able to help:

I install ML as instructed but when the system reboots it says there is no bootable device. I know for a fact that the system has a 1TB SSD drive since the OS was installed on it!

I go into the BIOS but I cannot get it to boot from the SSD drive. I have googled everything I could and tried different options.

I use the USB Drive to boot into a working ML and I can see the hard drive!

I have tried everything that I can and I just don’t understand. It was at one point working but since I have on multiple occasions installed different versions of Ubuntu and different versions of ML (each time formatting the drive) but now nothing boots!

I think I misunderstood what was written until the next morning.
It says “There should be no differences between Linux Mint 19 and Linux Mint 19.3.” so I assume that the instructions were written for a different version but it still works for the older version.