When Indian or India People are saying do one thing it's because they cannot usually speak proper English.
Do One Thing means to tell the person to go do something such as go to the store and get something.
Do one Thing please and go to the store and get a voucher card.
That is a saying from a scammer I've heard say that when India IRS Scammers are scamming people out of money.
The India Scammers in the India Scam Call Centers are most commonly known for saying "Do One Thing".
Not everyone India Person says do one thing but some of them do and it's mostly the scammers trying to speak English to the Americans they are trying to scam out of money.
Some India people also say "Do the Needful".
Neither is really correct but it's about all they can learn to say when they are first learning English Speaking.