Can I avoid an obstacle or what will happen if I avoid an obstacle? Is there a penalty?

The Spartans do not avoid obstacles, they overcome them!

We strongly discourage athletes from avoiding obstacles without at least attempting to complete them.

However, if you simply can not pass on an obstacle, there is a penalty of 30 BURPEES that you must perform before you can start the race again.
This could amount to a significant number of BURPEES, so it is best is to motivate yourself to pass the obstacle instead.