Lost between two macOS versions after failed upgrade to macOS Mojave I have no experience whatsoever with macOS (Linux user).

I got an old mid 2012 13" MacBook Pro, which was running OS X Mountain Lion 10.8.5. I tried to upgrade it to macOS Mojave, by launching Mac App Store, and searching for Mojave. After some time, the process stopped with the message "requested range not satisfiable".

Now I am lost between the Mountain Lion world and the Mojave world, because:


About this Mac says Version 10.8.5. I also can't install e.g., Xcode because the OS is too old.
The Mac App Store says that the Mojave upgrade is installed, hence the install button is disabled.


How can I properly upgrade to macOS Mojave? Restarting the device does not change anything.

I want to develop and publish iOS apps, so I require a recent version of macOS.

Progress (I)

I installed El Capitan, Sierra and then High Sierra (yes, one after the other; it takes several hours).
It was quite difficult to find these updates as the App Store-Search does not show them. I had to google it, find the Apple-page which contained a link (under 4. Download macOS XXX) which is opened in the App Store. from there on, updating worked.


El Capitan
Sierra
High Sierra


Now I have High Sierra, but I'm still not able to update to Mojave. I get the same error ("requested range not satisfiable").

Progress (II)

I think I know the problem. When I try to create a bootable installer (sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume), it says 


  To use this tool, you must download the macOS installer application on a Mac with 10.12.5 or later, or El Capitan 10.11.6. [...]


Ok, I have downloaded the Mojave installer when the OS was still Mountain Lion, so I understand that message. Problem was that I could not "re-download" it. I had to delete the /Applications/Install macOS Mojave.app/ -directory by hand. Renaming or moving to a different place was not sufficient. After that, I could start the Mojave-Download in the App Store as expected. The installation was successful after all.
It appears that upgrade to macOS Mojave hasn't gone through and you are stuck in a state where you are unable to run the macOS Mojave installer via Mac App Store.

You can try locating the downloaded macOS Mojave installer locally under /Applications directory. The installer (if available) should appear by the name Install macOS Mojave.app. If you able to locate it, try running it manually.

If that too fails, you have a couple options:


Install macOS Mojave via Internet Recovery. Instructions can be found in the Apple Support article, About macOS Recovery.
Download macOS Mojave installer on a different Mac, create a USB bootable installer and install using it. Instructions can be found in the Apple Support article, How to create a bootable installer for macOS.


If you are installing using USB key, you'll need to hold down Option key while booting your Mac and choose the plugged-in USB key to run the installer. It would be advisable to erase the built-in disk and perform a fresh installation.

Once you have macOS Mojave installed successfully using either one of the above approaches, you can install Xcode normally via the Mac App Store.

As mentioned that you wish to develop and publish iOS apps. Publishing iOS apps requires current or just prior release of macOS with recommendation to use the latest iOS SDK and current version of Xcode.

As your MacBook supports running macOS Mojave, it's recommended to install it along with Xcode 10.2 (current as of this writing).