Answer:
Neither Alaska nor Florida HAS a state income tax however residents in those states must still pay federal income tax.
Step-by-step explanation:
According to subject-verb agreement in concord, when the word "neither" is used to compare two things, it is proceeded with "nor" which is correct in the given sentence, and is a singular verb.
However, the given error in the sentence is the use of "have" which is incorrect because the word "have" is a plural word that is used along with plural words.
Therefore, the correct word to be used in the sentence is "has".