There are positions in which a player is unable to carry out a
turn. Such positions are extremely unlikely to occur in actual games. The
diagram shows a small, contrived example.
This is why the rules need a declaration about stalemates. Given the fact
that draws are otherwise impossible in SLITHER, it is most natural to regard
unability to move as a loss.
The official rule is that you must pass your turn if you have no moves available (but must make a move if you have one).