A dream that you are lying can be a sign that you feel that someone has been lying to you, or that you are feeling guilty about being dishonest in waking life.
If you dream that someone else tells you a lie, you may feel that someone has done you wrong in waking life.
The lie in your dream could also highlight something you would like to change in waking life. For example, lying about being a success could reflect an actual wish that you are successful in waking life?