I don't think you succeeded in not converting your SSD to APFS but there was something wrong with the SSD, and it couldn't be converted. Now you will have to reformat your SSD. If you wish to have the SSD formatted HFS+ instead of APFS then see the following: How to Skip Converting to APFS When Installing macOS High Sierra and Is It Possible Not to Convert to APFS When Upgrading to High Sierra?.
Since you appear to have a functioning Recovery HD then you can do the following:
Install El Capitan or Later from Scratch
If possible backup your files.
- Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:
- Select Disk Utility from the Utility Menu and click on Continue button.
- When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
- Click on the Erase icon in Disk Utility's main window. A panel will drop down.
- Set the Format type to Mac OS Extended (Journaled.)
- Click on the Apply button, then wait for the Done button to activate and click on it.
- Quit Disk Utility and return to the Utility Menu.
- Select Install OS X and click on the Continue button.
This will install the version of OS X you had installed.