A castsequence needs more than one element:

/cast [@targettarget, help] reset=combat rejuvenation, null

Would cast a rejuv once at the beginning of combat or

/cast [@targettarget, help] reset=target rejuvenation, null

would cast a rejuv whenever you switched to a new target

If you're trying to do dungeons though you would probably be better off taking the druid feral and doing a more traditional set up.