My Mac book pro won’t start up after an update and I can’t find my Macintosh utility disk to erase

You will want to command-option-shift-R to startup your Mac, and go to Disk Utility. Make sure you are connected via a Thunderbolt-2 to ethernet adapter to your modem when you do this via ethernet.

Under Disk Utility, View all. And then select the top most partition to erase. Once erased, run the Mac OS installer.

Alternatively if you made a bootable Mac OS clone of Mac OS 10.12 through 12.0 just clone it back using that clone's own disk utility. Option key booting the clone let's you make an exact replica of anything from Sierra to Monterey on a 2015 Mac.

Bring the Mac to an Apple Usergroup meeting where someone with the right OS on a hard disk may already be available. You might also find an Apple Authorized service center that has the older OSes on their recovery tools.

Won't turn on at all? If your Mac doesn't turn on - Apple Support

Won't start up all the way? If your Mac doesn't start up all the way - Apple Support