Can you please explain me the difference in meanings between these two sentences?
This would not have been happened if you were not there at that time.
This would have not been happened if you were not there at that time.
Neither sentence is grammatical English or something that a native would typically say, so it is pointless to argue over what they might mean. I don't see a reason to correct them differently.
“*This would not
have been happened” is a passive construction. But happen is not a transitive verb, so it cannot be used in the passive voice. It is likely that the author of this sentence intended to write “This would not have happened”, i.e. the event mentioned previously would not have taken place.
In addition to the previous defect, “*this
would have not been happened” is incorrect because the negation cannot be placed where it is. The negation word not is normally placed after the first auxiliary: “this would not have …”.
Furthermore, the second part of the sentence is in the wrong tense. “This would have happened” is a past conditional. The part of the sentence introduced by if is a past hypothetical. It needs to be in the past tense because the time of the action is in the past; in addition, the hypothetical aspect is expressed by a past tense, which means the verb needs to be in the pluperfect tense: “if you had not been there at that time”.
A correct sentence resembling these two incorrect sentences is
This would not have happened if you had not been there at that time.
I can't think of a correct sentence with a different meaning that could be what the author was trying to express.