It appears you have a Fusion Drive setup which combined the slow spinning hard drive with the fast SSD. However, the Fusion Drive had the newer APFS file system on it and you are booting into an older version of macOS which doesn't know anything about the newer APFS file system. You have two options depending on your objective here.
If you want to try to keep the existing data and reinstall macOS over top of itself, then you will need to boot into the same or higher version of macOS that is installed currently on the system. You can try booting into Internet Recovery Mode using Command + Option + R to attempt to access the latest online macOS installer compatible with your Mac, or boot from a bootable macOS USB installer. Unfortunately some Macs may boot to the oldest online macOS installer regardless of the keys used to access recovery mode.
Or, if you want to perform a clean install of macOS, you can try to access the most recent online installer mentioned above, or you can use the current installer you are accessing to recreate the Fusion Drive (you may first need to erase both the HD & SSD...not sure which may involve first deleting the "APFS Media" item).
Here is an Apple article with instructions for recreating the Fusion Drive from scratch (this destroys all data on the two drives):
How to fix a split Fusion Drive - Apple Support
If your Mac can still boot normally to the internal drive, then I highly recommend you first create a bootable macOS USB installer while you still can using the instructions in this Apple article:
Create a bootable installer - Apple Support
FYI, there have been a lot of user reports for the past few months reporting that it is no longer possible to reinstall macOS through recovery mode, but I have also seen a couple posts where some few may still be able to do so. Better safe than sorry, so create a bootable USB installer if you still can, otherwise you will need access to another compatible Mac to do so.
