“Hardly” already has a negative meaning (it means “almost not at all” or “only with difficulty”).
So if you say “can’t hardly,” you’re using a double negative (can’t + hardly), which is grammatically nonstandard in English and can make the meaning illogical — or at least informal/slangy.
Examples:
If you’ve ever written “I can’t hardly wait” and wondered whether it’s right, you’re not alone. The confusion usually comes from mixing a negative (can’t) with a word that already has a negative meaning (hardly). is it can hardly or cant hardly free