Can't upgrade to El Capitan 10.11 on Macbook Pro (2015)

Due to Adobe CC 2018 requirements, my Macbook Pro has to run on a version of OS newer than it currently has.

At the moment it is on Yosemite 10.10.5 (due to software reason I haven't upgraded yet), but now it needs "macOS 10.11 or later" to run Adobe 2018.

Once I found the El Capitan page on the AppStore and click on "Get". I get an error stating "We could not complete you purchase. This version of OS X 10.11 cannot be installed on this computer" The learn more button takes me nowhere. According to the El Capitan requirements, the Macbook Pro should be 2007 and older.

Why can't I upgrade?

I do not want to upgrade to Sierra (again, due to some softwares on the computer), but I really need a newer version of macOS.

Any help is appreciated.