How do you upgrade from Mountain Lion to Big Sur - keep getting 102 error messgae

Mountain Lion is only available for 2013 and older Macs.

Go to Apple menu -> About This Mac to verify the age of your Mac.

Big Sur is only available for these and later machines:

MacBook early 2015 8,1 (shipped with 10.10.2)

iMac 2014 14,4 (Shipped with 10.9.3)

MacBook Pro Late 2013 11,1 shipped with 10.9

Mac Mini 2014 (7,1) shipped with 10.10.

MacBook Air 2013 6,1 shipped with 10.8.4 (Mountain Lion)

Mac Pro 2013 9,1 (shipped with 10.9.1)

As you can see only the Air 2013 model is that shipped with Mountain Lion could run Big Sur.

So something is amiss with your identification.

Use http://checkcoverage.apple.com to check your serial number to find the machine age, and DO NOT post the serial number here.

Mountain Lion won't run on a 2017 Mac.

If you want Mountain Lion, you need a 2013 (prior to Mavericks release) or older Mac.

The 2012 may even restore it from Internet Recovery which is command-option-shift-R.

Any Mac that can run Mountain Lion can also run El Capitan.

Is there some reason you need Mountain Lion?

10.7 Lion 2011

10.8 Mountain Lion 2012

10.9 Mavericks 2013

10.10 Yosemite 2014

10.11 El Capitan 2015

10.12 Sierra 2016

10.13 High Sierra 2017

10.14 Mojave 2018

10.15 Catalina 2019

11.0 Big Sur 2020