Reclaim Free Space Partition in High Sierra
The easiest way is to reset everything and start over.

In the recovery partition (which you appear to be in, but if you aren't, use that) click the View button (in the window, upper left hand corner) and click "Show all devices."

This should show a hierarchy instead of a flat list in the sidebar. Select the top-level representing your hard drive. Mine says "APPLE SSD SM1024L Media". Your computer will likely say something else.

Click the Erase button. Enter a partition name ("Macintosh HD" being the default). Set it to APFS and GUID Partition Map. (This will remove the Recovery partition, but you will still be able to initiate the recovery process via Internet Recovery, so it shouldn't matter.)

You can now exit Disk Utility and use the macOS installer to reinstall the recovery partition and macOS.

Note: If you experience an issue during the installation phase, I've noticed that sometimes the installer doesn't like the disk being APFS at first. Wipe the disk again and select "Mac OS Extended (Journaled)." It will be converted to APFS during installation.