How can I play online when it tells me I need to buy gamepass or Xbox live in rainbow six siege

Yeah sometimes, or when im playing Rainbow six siege, and i get a msg telling me how bad i am :'D

You would need a Gold subscription or Game Pass Ultimate (which includes Gold) in order to play online.

Rainbow Six Siege was free to play for gold members only for a period of time. In the end of the said period you have to purchase the game in order to keep playing

As stated in the xbox wire:

Haven’t had a chance to experience the tactical action in Tom Clancy’s Rainbow Six Siege? Now’s your chance! Play Rainbow Six Siege for FREE on Xbox One all weekend from July 28-31. During the week, you’ll have the option to purchase the game online from
the Xbox Store at a discount and continue playing. You will also get to keep your Gamerscore and achievements earned while playing, so draft your Xbox Live friends and siege on.

news.xbox.com/.../rainbow-six-siege-free-play-days

Rainbow Six Siege was free to play for the past weekend. It was only available for that free period once that has finished you would need to purchase it

Starting August 16 until August 20, 2018, Xbox Live Gold and Silver subscribers will be able to play
Rainbow Six Siege free on Xbox One all weekend long.

It’s on gamepass, the one he tried is rainbow six siege