Why did Rav tell Rav Acha to sleep in kilah if he new that he won't fufill the Mitvah of Sucah. He could of just slept in his house?
Good question. It seems that Rav Acha did not have another house to go into (a possibility which is plausible, as we find in the Gemara on 48a; in this case, perhaps Rav Acha was away from home at the Yeshiva, learning Torah from his rebbi, Rav), and therefore Rav permitted him to sleep inside of the Kilah in order to protect himself from the discomfort caused by the bugs. Alternatively, Rav Acha did not want to have to move his bed out of the Sukah for the night and then back again the following night (when the bugs subsided).