Humor and humour are two spellings of the same noun or verb. The spelling depends on the type of English.
In American English, you use “humor,” so “sense of humor” is correct.
In British English, you use “humour,” so “sense of humour” is correct.
The same difference applies to similar words, such as “behavior or behaviour,” “labor or labour,” “favor or favour,” “favorite or favourite,” “color or colour,” and “honor or honour.”
A QuillBot grammar check can help ensure you’re using either “sense of humor” or “sense of humour” consistently.