Trying to Upgrade X El Capitan to macOS Mojave

Check here, How to upgrade to macOS Mojave - Apple Support to see if your iMac can upgrade to Mojave, if it is not in the list of supported Macs then you can probably still do this, How to upgrade to macOS High Sierra - Apple Support

I would also have a word with him indoors as to why he is buying you second hand items.

If you want to update it ON the computer, update to Sierra, then do the other Sierra updates, you’ll get to High Sierra, update through High Sierra, and then update to Mojave. You will be in Mojave for a short time when macOS Catalina comes out this fall.

What message do you get when you attempt to download Mojave?

It's not necessary to upgrade to either Sierra or High Sierra to install Mojave.