Hi! “delicious” is a non gradable adjective. You don’t use it with the adverb “very” because “delicious” already means “very tasty”. With non-gradable adjectives you can use absolute adverbs such as “absolutely”, “incredibly”, “truly”, “utterly” or “completely”. If you are not sure whether or not an adjective is gradable or non-gradable you can use “really” to intensify the meaning. So “really tasty” and “really delicious” are both OK. 😊