Can i buy jr east pass in japan?

i think i can buy jr east pass in japan but i don't understand this sentence (in JR-east website)
'When purchasing, it is necessary to present your passport and return flight ticket etc. (including "e-ticket customer coupons").
what is "e-ticket customer coupons"?

The e-ticket for your flight, I would assume.

The coupons refer to each piece of paper that exists in your ticket - one for each flight. But that was during the days of paper tickets.
These days virtually every airline uses e-tickets - which is great because replacing a lost paper ticket is truly a nightmare. They just need proof of your international ticket, since it is a fare made for international tourists, not residents in Japan.

JR would like to confirm you are a temporary visitor to Japan, returning to abroad after a short term stay.

So they are requesting to present printed air ticket booking confirmation.