RBCs are responsible for delivering oxygen to the body via hemoglobin.
As a result, they are designed to incorporate the haemoglobin in order to supply it, and hence they lack cell organelles such as the nucleus and mitochondria.
However, they require energy just like any other cell, but their energy requirements are relatively minimal and may be met purely by glycolysis, which does not take up as much room in the RBC as a cell organelle would.
Mitochondria:
Mitochondria are membrane-bound cell organelles that produce the majority of the chemical energy required to fuel the cell's metabolic activities (mitochondrion, singular).
The mitochondrial energy is stored in a tiny molecule known as adenosine triphosphate (ATP).