I think you may be missing what it is actually trying to tell you.
In order to upgrade to Mojave, you need to have one of the following machines at the minimum. I think you may meet this first requirement, IF your MacBook Pro is a Mid 2012 model:
MacBook (Early 2015 or newer)
MacBook Air (Mid 2012 or newer)
MacBook Pro (Mid 2012 or newer)
Mac mini (Late 2012 or newer)
iMac (Late 2012 or newer)
iMac Pro (2017)
Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)
But that is not all. In additional to the above hardware requirements, there are also software requirements; that being that you need to upgrade your OS step by step from your current OS, all the way up to the last release of High Sierra, before you will be able to install Mojave successfully, at least so I have seen a few people state.
Following is a short list of previous versions of macOS, so that you can determine what upgrade path you need to follow:
10.7 - Lion
10.8 - Mountain Lion
10.9 - Mavericks
10.10 - Yosemite
10.11 - El Capitan
10.12 - Sierra
10.13 - High Sierra
10.14 - Mojave
I'm not absolutely certain, but I think you may possibly be able to skip a few versions after Mountain Lion, and maybe start at El Capitan or Sierra. But then you have to follow the path up from there, or you won't be able to install Mojave. It is a bit confusing, because different Apple pages -- and other websites -- seem to say different things about this issue; but I am certain that you cannot upgrade directly from Mountain Lion to Mojave.
You can learn more at the following URLs:
https://support.apple.com/kb/SP777?locale=en_US
https://support.apple.com/en-gb/HT201475
I hope the above is helpful to you, and that it resolves your issue. Please let me know. Thanks!