original HD died about a month ago

I have a late 2009 27" desktop, and the original HD died about a month ago. I got a new SSD from macsales.com, installed, made a bootable disk, and everything was going fine, except for a glitch with iMovie which is one of my most used apps. I was hoping that updating my OS would help. I ended up loading High Sierra.

I found out that Quickbooks wasn't compatible, so I wanted to go back to an older OS rather than spend the $400 that the crooks at Quicken want for QB's. Evidently the HD has been reformatted to this APFS stuff, which I have no idea about.

Now I'm absolutely stuck. I start the computer with the Command R buttons held, and have the option to load High Sierra from the internet. Every time, it goes through the whole 20 minutes of downloading and ends with an error message that says, "Could Not Create A Preboot Volume for APFS Install"

I tried restoring from my external HD via Time Machine, but that ends with, "An error occured while preparing the disk "Macintosh HD". Try erasing disk with Disk Utility or choosing a different disk to restore to. I tried erasing, but it's just a viscious loop.

If any of you can help me get my mac functioning with any operating system, I would be super grateful.