Fulfil and fulfill are two spellings of the same verb. The spelling depends on the type of English.
In American English, you use “fulfill” (with a double “l”).
In British English, you use “fulfil” (with one “l”).
Australian English mostly follows British English guidelines, so “fulfil” with one “l” is most common.
It’s important to choose one and use it consistently. The QuillBot Grammar Checker can help you with this.