Error: SECTOR XXXX is not closed?

what does it mean! how can I close it it highlights a vertex & there is no vertex properties so how can I fix this error. knowing that when I move the vertex the lines joined to that vertex move as well so what does it mean by open!!! tell me how to fix this.

Sectors should always be closed (unless in some exceptional hacks that you don't need to worry about) This means that all surrounding sidedefs should be on the same sector. The vertex that was highlighted is where it found a hole (where the sector is broken). Check the linedefs near that vertex to see if it's sides are on the right sector index.

This will all be a lot easier to fix when Doom Builder 2 comes out, it might even fix the sector automatically (with a click on the Fix button) when you do an error search.

What I'd do is delete the sector its attached to, and then add another sector over it. If you can find the leak, patch it up, or delete the line and put a sector in it.

I know this is bad advice, but it could be a temporary fix if expert jargon makes you go "lol wut?".