Incorrect spelling, explanation: it is a common mistake to omit one letter when there is a double in a word and that’s why the form disapoint is a common mistake, too. It refers to the fact that double letters are not heard in speech. Therefore, it is a mistake commonly made by both learners and native users of English.

Correct spelling, explanation: the Old French disappointen is the origin of a well-known modern English word disappoint. The root of the word, however, is appoint and usually refers to an introduction to something. The prefix -dis, which is added to the word, completely changes the meaning, giving it a negative reference.

Definition of disappoint:
1. verb – not to meet someone’s expectations, make somebody unhappy,
He disappointed everybody with his rude behavior, even himself.

Disappoint word formation:
Other common words that come from the same word family include disappointment (a noun), disappointed (an adjective), disappointing (an adjective). As an adjective, it is usually followed by the preposition with.
The expectations were huge, but the film turned out to be a disappointment to everyone.
I was so disappointed with his rude comments to the teacher that I told him off later.
I was looking forward to the holiday, but unfortunately, the weather was too disappointing to enjoy it fully.

Incorrect spelling