how can I buy logic pro X

This is another very annoying Apple app store behaviour.

You can download the 'latest' version your OS will support but with an annoying caveat. You must have already bought the app.

For example, I bought Logic X when it came out. If I re-installed 10.11.6 onto one of my machines as a clean install and went to the app store it would allow me to download an earlier version of Logic that would work on 10.11.6 as Logic would be in my 'Purchases'.

The way around this is to find someone you know and trust who is running a later OS version and ask them if you can log into their machine under your Apple ID. You can then purchase Logic X and once the transaction has gone through etc you can immediately delete Logic and sign out.

If you then go to your own machine Logic will show up under purchases and will allow you to download a previous version.

Annoying and tedious I know...

Does your Mac support OS X 10.12? What model and year is it?

Your Mac will support Mojave, and there's no reason to not upgrade to it. I'm using it on a 2015 15" rMBP just fine. Love it.

It only allows latest compatible app version if you've already purchased on what was then a curent version.

It is mental. If Apple can detect what OS you're running and not allow a download and they can provide the last compatible if you've already purchased, then surely it wouldn't hurt them to allow first-time buyers to be able to install last compatible. But they almost certainly won't. The reason being, of course, they don't make their money from apps. They make it from hardware sales.

I didn't say Macs, I said hardware. That includes iPhone, iPad and iWatch along with Macs.

On all of those platforms Apple makes its profit on the hardware not the software.

I wonder what the 2019 modular Mac Pro will bring. One thing I can probably say with 99.9% certainty... a hefty price tag.

App Store is the only way to do it. You could repost this question in Apple ID discussion, see what folks hanging out there suggest.

I see Danirie, is there a particular reason why you must use El Capitan? Your Mac is not that old, and Sierra isn't bad at all. Even Mojave, as BenB said above, is tolerable.

Danirie, normally when you purchase the Logic app, App Store should allow you to install the most recent compatible (with your current OS) version of Logic. What exactly do you see in App Store, may I ask? Could you make and post a screenshot?

Note: I was about to suggest what octopi said above, so +1 octopi (great minds think alike) :)

octopi, yes it is mental, and yes I agree that these little tricks are meant to make people buy new Macs. Though, according to sources, Apple is making most money by selling phones not Macs. With Macs it's caught in between the rock of having to maintain good quality of its products that last for years and a hard place of resulting sluggish sales. Still doesn't stop them from making something stupid like iMac Pro, which costs so much that no one in their right mind would buy it.