Why did olivia lie about the ring and why doesn't viola tell malvolio the truth?

I think you are referring to scene 2,

Malvolio catches up to Viola, with the ring he was instructed to give Viola by Olivia. Viola is surprised, since she left no ring with Olivia; Malvolio grows impatient with Viola's claim to know nothing of the ring, and he throws it down onto the ground, and storms off. Viola realizes that the ring is proof that Olivia has some affection for her as Cesario; she regrets that Olivia is in love with her disguise, as that will come to nothing, and also that she is in love with her master, but that she can do nothing in her present disguise.