"The cat is on the chair."
Translation:El gato está sobre la silla.
En la silla means "in the chair" where sobre means "on the chair" while it may be understood it could be misunderstood or imply the cat is inside the chair.
Well then, something has changed, because 'El gato está en la silla' was marked wrong for me. Not to worry. The main thing is to know when you are right, and not to stress about Duo's occasional oddities !
'En' does not only mean 'on'. In many exercises Duo translates it as 'at' eg, en la playa = at the beach