First of all check that you made your map with a ceiling, floor and at least 4 walls (no black void should be visible from inside the map, not even a little bit of it).
Next see that you didn't insert an entity so that it touches the outside void or that it's outside the map completely.
And also make sure that you don't have any brushes textured with CLIP or ORIGIN texture outside your map (or touching the outside void).