Correct spelling, explanation: the full form of 71st is seventy-first, so it cannot end with the suffix -th. All ordinal numbers that end with a digit one, two, or three (except eleven: 11th) must contain an irregular suffix. The others end with the suffix -th, so it’s easy to get confused. However, this rule should be kept in mind when creating ordinal forms. It follows that the only correct form is 71st, not 71th.

Definition of 71st:
numeral, a number that comes between seventieth and seventy-second
My grandpa won’t have his surgery soon – he’s 71st in line.
I just bought my 71st stamp.

Incorrect spelling, explanation: if the number ends with 1, the ordinal form must be written by adding the suffix -st to it. It’s an exception, just like 2 and 3, and that’s why many English users get confused. Before creating an ordinal number, always look at its ending. In this case, the number ends with first, so the only correct spelling is 71st, not 71th.

Incorrect spelling

Incorrect spelling