I can not buy the Xbox Game Pass for PC

If i understand the problem, you want to buy them both an Xbox Game Pass subscription to play on their respective PC's ? Buy an Xbox Game Pass PC sub for each or a Ultimate Game Pass. Log their account on Xbox beta app on PC and buy them the correct sub.
Or use xbox.com with their account correctly sign in and buy them Xbox Game Pass for PC.

Regarding Game pass on Xbox. If you have a game pass you can set your box as homebox.

Then, any other account can share your subscriptions, games, and apps on that Xbox. You can Buy Ultimate and then one of your sons when logged with your account on The PC can have a game pass. Then you must buy a game pass for PC to another son, but with
his account. If you want both of them to have their own PC game pass, then you must buy a game pass for each account, but with their accounts. Homebox

Since you are saying Xbox App, can I assume that you are trying to play on a PC? If you are, did you purchase Game Pass for PC or did you buy the regular Game Pass which is only for the console?

For PC gaming you need either Game Pass for PC or Game Pass Ultimate.

If i buy xbox pass and download ark for free, will i able to join steam ARK servers or EPIC ark servers, plz let me know so that i can buy it.

The Xbox Game Pass will only allow you to play with the Xbox Game Pass for PC, and Xbox versions.

If you only have standard Game Pass for console then you'd either need to buy the game for PC as well or buy Game Pass Ultimate (to have both console Game Pass and PC Game Pass for free). Not all games can be played on both console and PC even if the game is available on both.