And why not "这些鱼很贵“ ？
Why must we use 非常 in this example, instead of 很 ？
These are synonymous! "这些鱼很贵" should be accepted.
Both are acceptable ways to say it in Chinese. We don't actually must use it, it's just Duolingo makes mistakes.
非常 is just a variation of 很, but 非常 is implies something nore extreme. How I put it into words is like saying "the fish is freaky expensive."
Zhexie = these
Is 这些鱼太贵 also a valid way of saying it?
太 implies that it's too expensive. 这些鱼太贵 = These fish are too expensive.
Here you are saying that these fish are too expensive
Why is there no actual use of a verb in this sentence?
In Chinese, adjectives can describe the subject without the need of a verb.
"很" is often used to link the subject and the adjective. If there are adverbs like "非常", "太", then "很" is not required