What does "I can't never" mean?

Two contradictory interpretations are possible.

If the speaker is erudite and elderly it means, "We can come here, though perhaps seldom." This figure of speech is known as litotes: the two negatives cancelling each other out. In literature it is often encountered in constructions like: "The bishop was not, I think, dissatisfied with my speech". Used like that litotes seems old fashioned, but in "It's not OK" and "That's not bad" it is more serviceable.

In Cockney and other dialects it means the opposite of what it seems to mean. "We can't never come here" is an emphatic form of "We can't ever come here". This non-standard usage is known as a double negative. [This is a poor example though. "We can't never come here again" or "We can't never go there" would make more sense.]

The formulaic protest "I ain't never done nothing!" - I haven't done anything - is still heard today, despite being overused in films of the forties and fifties to signal 'Cockney character'. ("Gor blimey guv'nor!" performed the same function.)

Without context it's impossible to say which interpretation is appropriate.