"Fyddi di mewn ymarfer heno?"
Translation:Will you be in a practice tonight?
Earlier the translation 'training session' was given for 'ymarfer' but it's now marked as wrong
Duo has been busy making corrections to the beta draft of the course, I expect.
As a noun, ymarfer generally means 'a practice, an exercise', etc. As a verb-noun it means 'practising, training, rehearsing'. hyfforddi is another verb-noun meaning 'training, coaching'.
'A training session' would be something like sesiwn hyfforddi, sesiwn hyfforddiant, sesiwn ymarfer or similar.