Felixxx The neighbors must have having a party Not correct. Is this what you mean? The neighbors must have been having a party.
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FelixxxThe neighbors must have having a partyNot correct. Is this what you mean?
FelixxxI mean it is impossible that the driver didn't see the red light.That would be The driver must have seen the red light.
FelixxxThe driver can't havent seen the red lightThis is wrong because it is a double negative; it may confuse the reader. You can convey the intended meaning this way: