Convert to APFS

Non-Destructive APFS Conversion

Apple provides non-destructive methods for start-up disk drives for APFS conversion, and hence no data (files or folders) will be lost. On the other hand, APFS formatting will erase all the files and folders stored on the external hard drive. If not backed up earlier, APFS formatting on the external drive will cause data loss. Even with the start-up drive, one may delete or lose files from the APFS formatted media.

How to Upgrade to APFS if Not Done in macOS High Sierra Installer - The Mac Observer

Always advised to have a current backup plan in place, especially upgrades, updates, file conversion— in this way you can fall back with no lose of data if something goes wrong. You can google "3-2-1 backup strategy" for more info.

How to create a boot clone

Not sure about this but I don't believe you can convert HFS+ to APFS non-destructively. I just checked on my machine and find the same constraint. I am not able to convert my HFS+ volumes to APFS. You will need to reformat the SSD from scratch as APFS.

Install El Capitan or Later from Scratch

If possible backup your files.

1. Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the new SSD volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Install OS X and click on the Continue button.

This will install the version of OS X you had installed.

Is yours a non-Apple SSD?

Some SSDs, particularly some from OWC, cannot be converted as the devices report themselves to the system as being regular hard drives rather than SSDs.

OWC's Aura SSDs Not Working With macOS High Sierra, Fix Coming Soon

I have no idea whether OWC ever provided a fix.

Interesting William Kucharski, never heard this. Seems a work around however for those poor souls is install High Sierra using HFS. Once the installer is in the Applications folder, users can launch from the command line:

/Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/startosinstall --converttoapfs NO

Not the case here however.

The OWC Aura is a PCIe-based flash SSD. The 2010 MBP is a classic 2.5'' form factor.

cheers !

How is the SSD currently formatted? It may already be APFS. Please tell us exactly what you are doing and how you are doing it. We are not mindreaders.