“The posture should be stable and pleasant” ~ Patanjali (Yoga Sutra 2.46)
Think of yoga and you probably picture someone sitting in the famous cross-legged Lotus Position. But not everybody is capable of sitting that way. Fortunately, being able to sit in Lotus Position isn’t really necessary for a successful yoga practice.
A proper sitting posture boils down to two essentials. First, the spine needs to be straight. This allows for a deep and complete breath. Second, the posture has to be comfortable, otherwise you’ll be distracted by sore buttocks, aching joints and weary muscles.
So it’s OK to sit on a chair, as long as you maintain a straight spine and sit comfortably.