I have been told that it is wrong to say "very incredible " and "very fantastic". Could you tell me why?
There are certain adjectives, called absolute adjectives, which allegedly should not be modified, because logically ( but not grammatically) there is no way to modify them. Logically, you can't be be very dead, very pregnant or very unique. I can give you a perfectly logical argument (and I admit that an argument is either logical or it's not) that our founding fathers couldn't form a more perfect union, and that you cannot be more specific because you either specify something or you don't. English is beautiful, but not logical, so that the answer to the question is that when you modify an absolute adjective, you should know that those of a logical bent will fuss. I won't agree with them not now, not never, not noways. Do not count up the negations to check the parity to see what I mean. Here, https://www.merriam-webster.com/words-at-play/very-unique-and-absolute-adjectives, is a defense of might point of view by Merriam Webster.
To my knowledge, it is not grammatically incorrect however saying "very incredible" or "very fantastic" is both redundant and uncreative. The word "very" is not bad but it generally leads to weak writing because it is often used to strengthen a word rather than finding a stronger word. "He ran very fast" is a weaker way of saying "he sprinted" (Edit: "he ran quickly" is a closer alternative) just like saying "that was very good" is weaker than "that was incredible" or "that was fantastic."
"Incredible" and "fantastic" are already strong words and the use of "very" only serves to weaken their impact.
To what extent we'd say "wrong" is debatable, but if something is incredible then it is impossible to believe it. If two things are impossible to believe then one cannot be more impossible than the other; it's an absolute. As an absolute it makes no sense to say something is very incredible; it's either incredible or it isn't.
Fantastic is similar in that it means something is not related to reality. It's a bit more defensible though to say that one thing is further departing from reality than another.
In figurative use though, it makes perfect sense that something described as incredible is more so than something else described as incredible since such figurative uses do not literally mean something is impossible to believe.
But in doing something with the word in a figurative use that cannot be done meaningfully with it in a literal use, it moves further from the literal meaning and figurative senses get their force and vitality from evoking the literal meaning.
As such, while it can be defended as a valid figurative use, it has less power that way and generally we use very to add force, not reduce it. So even if its not "wrong" as such, it may still be worth cutting out the very.