He probably used trade override. And it still would not let you trade for Cousins even with Lillard included.
There are a few superstars who are easily obtainable but Boogie is on untouchable list (you cannot get him, LeBron, Curry, Westbrook, Harden, Davis for any package at all). You can get DeAndre Jordan very cheap. You can get Embiid without giving up CJ or Lillard. Noel you get for Turner + 1st round pick + 2nd round pick or Crabbe plus two 1st rounders. Noel works well in the line up. Nurkic you get for next to nothing. Best of all, you can get Paul George for the right package - still hasn't managed to do it for Blazers because I never fancy letting CJ go but it must be possible. I got him at Boston and Philadelphia.
If you are playing someone other than Blazers and you are looking for a point guard, there is lots of them: Conley, Thomas and Walker you get for a good price.