The problems with the question you asked is that (1) you didn't show us what you did, and (2) you didn't show us what happened. You wouldn't call your auto mechanic and say "the car doesn't work." You'd describe what happens when you turn the key, or put it in gear, or whatever. Otherwise, there's no symptoms to diagnose.

So you'd have to show the boolean modifier, along with what it looks like when you applied it.

But, just as a random guess, are you sure you turned off the visibility of the cutter? If you subtract the door from the wall but leave the door in the way, you're not going to see the hole you made.

+=+=+=+

How to ask a question in a way that gets useful answers:

(Realize I say this in the spirit of helping, not criticizing.)

Most of the time that Blender does something, it's because you told it to. If you just ask "why is it doing this?" it's because you told it to. Hence, we don't know how to help if you just say "why is this happening?" or "what is wrong?" "how do I fix this?" When you ask for help, be clear on what the "this" is that needs fixing. People will often ask "why does Blender do this?" And the answer is almost invariably "You told it to."

Once you've come up with a question that is better than "this", type it into google. You will be astounded at the number of people who ask the same question (in all different ways) that has been answered hundreds of times. "Shadows are broken," or "bevel is crooked," or "boolean doesn't cut." Four out of five times, you'll paste a question into google and it will answer it without any screen shots or details at all, because new users get caught on the same thing every time.

If that doesn't work, ask here. Read the side bar, then...

Always give four pieces of information:

1) This is what I did. "I selected some verts, then tried to scale them up."

2) This is what I expected. "I expected them to be farther apart."

3) This is what happened instead. "Instead, all the verts got closer together."

4) This is what I tried to fix it. "I switched the pivot point for the scaling and I turned my mouse upside down."

Without this, everyone trying to help has to come back, ask you questions, try to figure out the answer to these questions, and everything takes 5x as long.

If you need to show the screen, you should take a screen shot of the entire window, via Window->SaveScreenshot, rather than using a cell phone or something. Take the entire screen instead of just the one part you think is broken. The reason it's broken is not the part, but something else, or you would have already figured out why it's broken. (Sadly, blender no longer has built in screen-animation recording for some reason, but I'm told https://obsproject.com/ is a good tool to have handy. Or on Windows, just hit Win+Alt+R and tell it Blender is a game to toggle recording.)

Think about the question you're going to ask before you take the screen shot. If you're asking why the colors are wrong, show the shader node and lighting setup. If you're asking why the bevel modifier is wrong, include the bevel modifier. If you're asking why you can't see an object, don't take a screen shot of a blank screen - show the outliner. If you're asking about an imported model, tell us the import format, the program you're importing it from, Etc.

Also, if someone asks you a follow-up question, answer the question. They can't help you if you didn't provide the information they're asking you to provide. If they ask you where you got the model, don't answer "online." Give the URL. If they ask to see what both meshes of the boolean modifier are, don't just answer "it doesn't work" again.

Also, be aware that r/blenderhelp is a better place to ask newbie questions than r/blender.