Correct spelling, explanation: th is the correct ending to use in 26 as an ordinal number because in the full-word form twenty-sixth this the last sound – th. Therefore 26th is the correct form. Other forms, like 26st, are incorrect, because only th can be used as a suffix – 26th just like twenty-sixth.

Definition of 26th:
numeral, the ordinal number between 25th and 27th
We have to wait for a while since we are 26th in the line.
Lucy’s friends are organizing a surprise party for her 26th birthday.

Incorrect spelling, explanation: this form is incorrect because st is not the right ending for this ordinal number. For ordinal numbers we use the endings from their word forms, so the ending for twenty-sixth is th, therefore the correct form is 26th. Because of this word form, 26st cannot be correct, and the correct form is 26th.

Incorrect spelling

Incorrect spelling