what do " can never " and " could never " mean?

"Can never" is absolute, while "could never" is conditional.

"After all his tickets and moving violations, Jim can never again drive in California." -- Jim cannot drive in California. There is no possibility that this will change.

"I could never sky dive. I am afraid of heights!" -- This is not absolute. I am afraid of heights, and so could not see myself sky diving. However, there is a possibility that that could change in the future. This is different from saying "I was in a major accident several years ago, and can never sky dive."

"Can" and "could" express ability. "Never" means "at no time." "I can never do this" means, "I am not able to do this at any time." This sentence is factual, based in reality. It might be the speaker's opinion, and reality can change. For example, a person might say, "I can never swim" because there is no pool or lake nearby. If someone builds a pool, he/she can swim now.

"I could never do this" is hypothetical, not based in factual things. We use this type of sentence to talk about imaginary or unreal events. You imagine yourself in a situation that is not currently happening, not real, and make a statement about it--"I could do that" or "I could never do that."

"Can never" means you've tried to do something and cannot. "I can never paint my nails without getting fingernail polish on my skin."

"Could never" means you would never try to do something because you cannot see yourself ever doing it. "I could never murder anyone."