How to straighten a twisted 2x6 deck board
Very little pressure treated lumber is straight, most of it is twisted, crooked, cupped, bowed, etc.  If you have just one bad board you got much better wood than most.
Just start at one end and try to straighten it in the process of fastening it down.  If you're using screws, it's pretty easy - the screws will pull the deck board into the joist.
Nails will too, but it's harder nailing when the board won't lay flat.  If you nail down the side that's in contact with the joist first, then use a clamp from the bottom of the joist to the top edge of the decking, that will pull it down so you can nail it easily without it bouncing all over.
If it's so bad you can't make it work, scrap it or maybe try to return it.