Monterey upgrade from Mojave on HD formatted for OS extended but

Boot into Recovery and open Disk Utility. Select that drive and look for a Convert to APFS… command in the Edit menu.

It may not be able to convert it for the same reason it couldn't create an APFS volume when you installed Mojave.

In that case, you may have to erase the drive and install Monterey from Internet Recovery or a USB Installer.

Only as some background.

From memory, somewhere around macOS 10.14.3 Mojave did the installation offer to convert the HFS Journaled drive to the newer and required APFS drive format.

This builds into the question, is the present installation of Mojave Fully Updated to the latest 10.14.6 and all the subsequent Security Update for Mojave ?

If Yes, the the pervious suggest maybe your only practical option

If No, you could avoid this issue by updating to the latest Mojave which may or may not presntthe Conversion to APFS