Install old app version on iPad 1st generation iOS 5.1.1 (Windows 10, year 2018 question) I've just got first gen. iPad with iOS 5.1.1 and reset it to it's factory settings. Then I installed iTunes version 12.4.3.1 on my Windows 10 PC. Both iTunes and iPad are connected to same account and both are authorized. Now I can search for apps in the App Store. 

I've already read tons of tutorials and advice how to install apps on outdated devices. I also checked this detailed answer of Chad's, but so far I find some app I need in App Store, I'm not more able to to do it. Install button on iTunes is now completely disabled.



And the same for any other app on the store.

Now I have now no clue how to handle it. I would be thankful for any advice.
As the text in the graphic says, you need to open the App Store on the iPad itself, not through iTunes on your PC.