If my flight is 8.40 am do I hv sufficient time to take the E21 from my apartment at Cheung Sha Wan?

Generally speaking, you need to arrive at the airport 2 hours in advance, which means that you are adviced to arrive at there at around 06:40.
The bus no.E21 takes around 65 mins from Cheung Sha Wan to the airport. So you must catch on the earliest schedule of this bus. Otherwise, you have to take a taxi to the airport directly.