Most people learn guitar while playing in a seated position. It is a natural and comfy position, which is good when learning the basics. It will also make the transition to playing while standing up feel awkward.
In this video, you’ll see some tips for using a guitar strap and playing in a standing position. It will help your posture and force you to concentrate on your chords with out looking at them. Great posture is what enables you to stand whilst playing guitar. To begin learning this essential element of being a great guitarist you will have to practice much more standing than sitting.
Be patient, it takes time to get utilized to standing while actively playing.