Incorrect spelling, explanation: the form fullfil is a misspelling. The problem may come from a different way to spell fulfill in American English and in British English (fulfil). The differences may come from the fact that the final l is usually doubled when adding the suffix, or because of the spelling of the word fill itself (usually differences in spelling particular American and British words derive from the fact, that Americans made their language easier and less complicated).

Correct spelling, explanation: this form is an American way to spell the word fulfil. The American way of spelling probably comes from the word fill (to make something not empty by putting something inside). It can also come from the spelling of words that derive from fulfil, in which the final l is doubled (for example fulfilling). Fullfil is simply a misspelling and doesn’t exist either in British or American English.

Correct spelling, explanation: it’s spelled this way outside North America, correct also in Canada.

Definition of fulfill:
verb, to meet one’s needs, satisfy expectations; bring to an end (as expected)
She wanted to fulfill her dream of becoming a ballerina.
This chocolate cake definitely fulfills my cravings for sweets!

Incorrect spelling

Incorrect spelling