Whenever you finish working on an area, you will need to 'bake' it before you can test it, or allow players into the area. Any time you make changes to an area, you will have to bake it again before you can test the changes you made will work.
To bake an area, go to 'File', and then 'Bake':
You will then get the following prompt:
Click on the "Yes" button if you are ready to proceed. The baking process is split into 7 parts and you will see a pop up dialog box, which looks similar to this:
Once the process is finished, your area is now ready to be played on. Please note that you will have to go through this process for each area. Any changes that you make to the walkable area you will need to rebake.
If you try and play with an unbaked area the player will be stuck where they spawn. You will also notice that the PLAYABLE area is covered by the Surface Mesh and does not extend to the surrounding NON PLAYABLE areas. You cannot paint WALK into these areas.
Placeables will cut out the walkmesh automatically during the bake process. This is especially useful for buildings and walls which you can use to strategically block areas of the map. Many objects like bridges and raised platforms will cut the walkmesh so the character can walk on the bridge or platform instead of the terrain below it. Note that if the bridge is the walkable, the area underneath the bridge is set to non walkable.
There is also a Baked button in the toolbar and when selected will show you white outlines that are cut out for placeables and walkmesh triggers.
Too many objects
Sometimes with densely packed placeables a tile region can get overloaded and will fail the bake process. You will see that by a tile that is completely unwalkable. There are two ways to fix this. Delete and/or move some placeables and try baking again. Or try converting some placeables to "Environmental Objects" (see placeables section) and surrounding by a Walkmesh Cutter. Walkmesh Cutters tend to be simpler for the engine to process a walkmesh from.
As you maybe able to work out from the picture above the whole tile is Non Walkable as there are too many placeables (6 graves and 3 fences).