My 13 year old daughter has had outside knee pain for 2 years what could it possibly be?

Lots of things. Lateral knee pain for 2 years is unusual and thus would warrant a thorough workup to find the diagnosis. The differential diagnosis is broad but would probably include infection, benign or malignant tumor, discoid meniscus, meniscus tear, osteochondral defect, patellofemoral syndrome or patellofemoral instability, or miserable malalignment.