Here’re some of the reasons why you can’t sort your cards by vote:
You’re not the owner of the board.
If you’re not the owner of the board you won’t be able to sort the cards by vote. To do that you’ll need permission from the owner of the board.
There’s an option on the setting of the board’s owner called “Allow anyone to edit”. Ask the board’s owner to select this option, after that you’ll be able to sort the cards by vote.
If you're the owner of the board unselect these options.
If you have selected the option "Hide vote count" or "Disable voting" the sort by vote feature won't work. Unselect these options, and after that try again to sort your cards by vote.