Correct spelling, explanation: to create a number form for ordinal number, we have to look at its full-word form. Here 6th is pronounced sixth, so we know that th should be the suffix. At the same time, we cannot use st (6st), because sixth doesn’t end with st (number first does).

Definition of 6th:
numeral, the ordinal number between 5th and 7th
I can’t believe Amanda is having her 6th birthday tomorrow! It seems as if she was born yesterday!
This is aunt Jackie’s 6th cat, she should stop adopting them, her flat is too small.

Incorrect spelling, explanation: 6st is a mistaken form because suffix st can be used only in numbers that end with st in full-word form; thus those which end with a number first. The correct form is 6th because sixth ends with th.

Incorrect spelling

Incorrect spelling