This volume is not formatted as APFS

High Sierra macOS 10.13 was the first version of macOS to support APFS if the computer had an Apple Original SSD Drive.

For sure in Mojave macOS 10.14 the Installer of that version of macOS would probably Offer the User to Convert the old HFS Journaled Drive to the the APFS Format.

You may consider the earlier suggest to make a Bootable Installer of Monterey and use it to install Monterey

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Monterey would have to be done on a computer that Qualifies to run Monterey.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

High Sierra is the oldest to support APFS. So you might at least have to run its installer from a bootable USB flash drive so the firmware is updated. Or do the whole High Sierra upgrade.