Could someone please explain me how and when to use these expressions? Is there any difference between the two of them? Is it something like "see you later/see you there/see you soon"?
不见不错 - there doesn't seem to be a reference to this phrase to be found... perhaps you could indicate where you saw this?
不见不散 - used to indicate a firm commitment to an appointment, in the sense that if we don't see [不见] each other then we will not leave/scatter [不散] (i.e. keep waiting).
If it is to another person and you want to make sure the other person waits for you then it means "don't leave before I get there".
If you are indicating that you'll be waiting for the other person then it means "I am not leaving until I see you".
If it is to a group of people then it means "no one's going anywhere until we all get there".
不见不错 is gobbledigook. 不见不散 means that you will in fact agree to meet someone, that you are really committed to it.
See you later is 再见 or 回头见.
不见不散, not see not walk, "wait for me， don't leave"
不见不错 is not an everyday phrase. Maybe it is meant to mean
见不到也没事, "If you don't see me (there) don't worry."