I have not tried any of the Zwift workouts and/or training programs. Failing a workout once is no big deal. It could just be an off day. With that said, not being able to do 6x5 minutes at FTP means you have your FTP set up too high. You should be able to ride anywehre between 30 and 90 minutes continuously at FTP. Five minutes at FTP should feel quite easy frankly. Also, not sure what 6x5 at FTP would achieve in terms of adaptation (my guess is nothing), but if that's what the workout calls for...