Can't install Monterey on MBP Mid 2015

The MacBook Pro (Retina, 15-inch, Mid 2015) would have shipped from the factory with OS X 10.10.3 (Yosemite) or some version of OS X 10.11 (El Capitain). Those predated APFS and do not understand it. Monterey would expect to be installed on an APFS-formatted volume, and I don't know if you could go straight from Yosemite or El Capitan (if you installed one of those via Internet Recovery) to Monterey in one hop.

If the Mac is now running Yosemite or El Capitan, you could try upgrading in several steps:

• Upgrade to Sierra. The Sierra installer is available in .DMG form, so you could use a modern Mac or PC to grab it from the Apple site (as a data file) for transfer to the MacBook Pro.
• Upgrade to High Sierra. High Sierra was the first version of macOS to use APFS on internal SSDs. The installer is able to convert HFS+ format volumes to APFS format volumes "in place." But you may need to be on El Capitan – or Sierra – before the App Store will work well enough to let you grab High Sierra.
• Upgrade to Monterey.

Update macOS on Mac - Apple Support

How to start up from macOS Recovery - Apple Support

Carlosilva wrote:

I've run the disk error tool and it doesn't say the disk is faulty.

If your computer can start up now, I would suggest making a bootable clone backup onto a spare external drive (using Carbon Copy Cloner or SuperDuper!) before doing anything further with the internal drive.

These days, doing Internet Recovery on old Macs, and installing old versions of macOS, may be a little bit more dicey than it once was. If you erase your only copy of macOS and have no working local copy to start up from, recovery may become a lot harder, if even possible.

I think Servant of Cats has the right idea, i.e., to upgrade in stages.

macerror 21

Mac OS error 21 (dsNoPk4): package 4 not present

suggests some software (like maybe APFS firmware package) is not available, so it can't install that version -- YET.

Firmware upgrades were installed with incoming versions of MacOS, and the firmware upgrades installed at Sierra, high Sierra, and Mojave were VERY important.

NB> firmware is placed in a private store on the Processor, not on any drive. Once firmware is upgraded, erasing/removing the drive used to install that firmware changes NOTHING. Your Mac is permanently more capable.