Of course it is possible to 'fail' a PhD course.
It is not an examination type of pass/fail though.
If the candidate is lacking in academic rigour, or
is lackadaisical in his/her research output, then
the supervisor will advise discontinuation of the
project. There is also a time-limit to produce a
80,000-100,000 word thesis, then it is submitted
to in-depth scrutiny by a panel of critical assessors,
often with a 'viva' to follow. it's a long, hard slog.