Classic Mode is one of the two main options when choosing the Game Mode during Game Creation. (The other main option is Sprint Mode.) It is always available as an option unless you have a Wild Mode with its own special win condition(s) activated, in which case it is not available.
The main goal in Classic Mode is to be on or past the finish space when win conditions are checked (when a turn ends or in-between Round Effects and a turn.) If 2 or more players have met this win condition simultaneously, they can all win in a tie. (This applies even if 1 of the players meeting the win condition is ahead of another.)
If the only player(s) on or past the finish space is being prevented from winning (and no other player meets an alternate win condition), the game will continue until a player on or past the finish space is not being prevented from winning when win conditions are checked.
Unlike some win conditions, a Classic win condition is not locked-in, so if you are on or past the finish space (including if you had Denied when win conditions were checked) and then you move backwards so that you are behind the finish space, you will have to reach or pass the finish space again to meet the Classic win condition.
Things that can prevent or delay a win include:
Things that can alter the finish space (also known as the finish line) include: