Can’t ( or couldn’t) have + past participle. :  This expression is used  to explain the opposite of ‘Must Have’ when it deals with the probability aspect. Examples will clarify the essence of this expression.

• Johny drove past me without stopping.  He can’t have seen me.( i.e probably he would not have seen me. We don’t say, he mustn’t have seen me) .   Now differentiate the expression ‘must have ‘ when used  in a positive sentence. Johny stopped his car on the way. He must have seen me walking and would have liked to offer me a lift.
•   ‘ He couldn’t have seen me. ‘ can be used in place of ‘ He can’t have seen me ‘.
• The drunkard walked straight into a wall.  He can’t have seen the wall.
• He couldn’t (or can’t) have met the minister yesterday. (maybe, the minister was very busy or otherwise engaged)

‘ Can ‘ with different degree of sharpness when used in the negative.

• We are unable to help you. ( the polite and preferred form of denial)
• We cannot help you. (more abrupt form of denial)
• We can’t help you.( the most abrupt form of denial)

Note :  ‘ We are unable to ‘ is often preferred in formal English to the more abrupt ‘ cannot’ and the still more abrupt ‘ can’t ‘.