Incorrect spelling, explanation: 26st is incorrect because the full-word form of this ordinal number ends with th (twenty-sixth; 26th), not st. Ending st will only be used in case of number first (for example 101st, 51st). Therefore the correct form is 26th, since the word ends with th.

Correct spelling, explanation: 26th is the correct form because the full-word form twenty-sixth ends with th, therefore this ending is also used when we write this word in a number form. This applies to every ordinal number, thus first ends with st, third with rd etc. This shows why 26th is an incorrect form and 26th is correct.

Definition of 26th:
numeral, the ordinal number between 25th and 27th
Boxing Day is on December 26th.
On his 26th birthday, we are having a big party at the seaside.

Incorrect spelling

Incorrect spelling