Aisha,
Sometimes hatching can be easy, sometimes a pain.
For instance in your drawing (Windows 10) if I tried to hatch the walls when zoomed in so that it would be easy to get my cursor inside the walls, it gave me the same error message.
If I zoomed out quite a bit so that I had all or most of that wall on screen, then it would hatch.
But regardless, here are various steps I take in trying to get hatching to work.
1. Check all corners and intersections to make sure they are closed. No gaps. Fillet command "F>Enter" is good for this. Extend, fillet, things like that.
2. Look for things that are intersecting your attempted hatch area. Like leaders, dimensions, blocks. Try freezing the layers those are on. But of course you can't freeze the lines that are supposed to be the border for your hatch.
3. Look for lines that are under or on top of your hatch border. For instance you have some short wall lines at the corner on top of or overlapping each other. Make lines that are just one line. Again, fillet helps here.
4. An object that intersects or is tangent seems to confuse hatching sometimes. Like a circle or arc that is tangent to a hatch border line. If the object is on a different layer, freeze it before hatching.
4. Sometimes zooming in or out can help, regenerating "RE", things like that.
5. Before 2018 Autocad seemed to require that the entire area to be hatched was visible on the screen to run the hatch command. Supposedly in 2018 they made that not necessary. But on your drawing when I was zoomed way in it didn't work. When I zoomed out it worked fine. Try zooming out so that the whole area to be hatched is visible.
6. Sometimes I will use a non-printing line and make a polyline border to be hatched. Then you can hatch that as an object. Invoke the hatch command>S>Enter and it will give you a square cursor for selecting objects. Hatching a closed border object with the "object" selection method is generally more reliable than filling an area with the crosshair cursor. It can still get confused, just not as often. It has to be a closed border. This method doesn't require the whole area to be visible on the screen.
GChapp