If Tom and Daisy from The Great Gatsby represent the corruption and materialism of the East, how can one explain how they are unadaptable to Eastern life?
"Tom and Gatsby, Daisy and Jordan and I, were all Westerners, and perhaps we possessed some deficiency in common which made us subtly unadaptable to Eastern life."
Your question can be interpreted in several possible ways. First, Tom and Daisy represent the materialistic and careless nature of Old Money society, not necessarily of only the East. The Buchanans' ability to live wherever they want to and buy their way out of any trouble they produce is associated with their moneyed background, not with their geographic location.
Secondly, Nick's comments about Westerners possessing a
"deficiency . . . which made [them] subtly unadaptable to Eastern life"
refers to his belief that the East was corrupting those who migrated there. He recognizes Jordan for the cheat that she is and perhaps reasons that the East has caused her to act in such a manner. He also assumes that his cousin Daisy's flippant attitude toward her husband's mistress and Tom's flaunting of his affair stem--in part--from the influence of Eastern culture. Nick, himself, never feels comfortable in the East and spends his time there as a rather objective spectator.
A better example of the East corrupting "Westerners" is Jay Gatsby. However, one could also argue that Old Money society is most responsible for Gatsby's downfall.