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".