At the start of The Legend of Korra, Korra can bend three elements: water, earth, and fire. As the Avatar, she is destined to master all four classical elements, but airbending remains the discipline she has not yet learned.
Korra is introduced as a powerful and unusually advanced young Avatar from the Southern Water Tribe. She learned waterbending from her native tribe and trained in earthbending and firebending before moving to Republic City. Her physical confidence contrasts with the spiritual patience required for airbending, which is why she must study under Tenzin, Aang's son and the only living airbending master at that point.
The missing element is not a sign that Korra is an ordinary bender; it reflects the Avatar's traditional training sequence and her personal difficulty with airbending's evasive, non-aggressive philosophy. Once she begins airbending, Korra can eventually use all four elements. She later also gains energybending and learns metalbending, but those are specialized abilities rather than additional classical elements.