Both options are possible in the conversational speech in some known dialects of English. Though, in the modern written speech the sentence You have done everything you can do is rare and historical. You have done everything you could do is more widely used, but, it needs a clause or phrase denoting purpose of the action. For example, You have done everything you could do to fulfil the task.Without such clause, the sentence You have done everything you could do is semantically somewhat incorrect.
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