All I know is this sentence has parallelism mistake but I don't know how to fix it!

Other people believe that after a nuclear war, the world, with radiation and where there would be disease, wouldn’t be worth living in.

In addition to parallelism issues, the subject and verb are interrupted by lots of words. That is usually a poor idea! Here's how I'd fix it:

Other people believe that after a nuclear war, with resulting radiation and disease, the world wouldn’t be worth living in.

How about this one?Thank you sooo much for your help!

Arkansas, the group feels, is the best place to be for several reasons: it is an unlikely target for nuclear attack; it offers plentiful supplies of food and water; a good climate.

After the colon, all three items in the series need to have the same structure. Currently, this is what's there:

~ independent clause
~ independent clause
~ a noun with an article and an adjective

How will you fix it? How will you make sure all three parts of that series match in their structure.

To correct the parallelism mistake in the sentence, we need to ensure that the parallel elements are structured in a consistent way. In this case, we can rephrase the sentence to achieve parallelism by using parallel verb forms. Here's one possible revision:

Other people believe that after a nuclear war, the world would not be worth living in due to radiation and disease.

In this revised sentence, we maintain parallel structure by using the verb form "would not be worth living in" and adding the parallel elements "radiation" and "disease" after the appropriate preposition ("due to") to create a clear and grammatically correct sentence.