Cant launch digital game

Open the Uplay client and log in to your account.
Hover to the game tile and a little arrow will appear on the bottom right corner of the tile.
Moving the pointer to the game tile and clicking on the small arrow that appears.
Click on this arrow and select “Verify Files” option.
Wait for the verification process to finish, run the game and check to see if the issue persists.

It could be the game's maintanence time. Try again few hours later. If it still does not work, try other games to see they work. If they work just fine, you might need to contact to Rainbow Six Siege support forum.

are you able to launch other games?

try soft reset https://www.youtube.com/watch?v=U8T9CMqmmK0

If there were games already installed on the console it sounds like you purchased a used console that someone had downloaded their games to.

Keep in mind that the license for the games go with the account that purchased them.

So what sounds like happened in your case is this.

1. You purchased a Xbox console that someone else had installed their games on.

2. The person that installed the games had set that console as their profiles home console which allows any other profile on that console to play their digital games.

3. You purchased a new console and were able to do a network transfer of the games themselves to the new console to save you from having to redownload them (would not of been able to redownload the games from the previous owner)

4. Since this new console is not set as that profiles home console you would not be able to play those previous games.

The reason you are able to play Black Ops 4 is because you purchased it with your account.

Since GTA V was already on the console when you purchased it the previous owners profile was the one that owned the game.

At this point if you want to play those games that were on the console when you purchased it you would need to purchase the games on your account.