"Navmesh needs to be rebuilt"

Whey I was putting in a navmesh, it said that it needs to be rebuilt “Navmesh needs to be rebuilt”.

I did not see any what to built the navmesh. How can I do this?

Thank you

Click Build → Build Paths
If it doesn’t work
check recast mesh agent radius value. It should be greater than 34

Yep and maybe you have to add one someday. Clicking “P” you can se where your Character can step on. Sometimes you may rebuild lighting to.