Why this happen?

The bedroom is a room.
The box is in the bedroom.
The box is a thing.
The box is closed and openable.

Problem. You wrote ‘The box is closed’ : but the property closed for the box is not allowed to exist, because you haven’t said it is. What properties something can have depends on what kind of thing it is: see the Index for details.

I dont see what is wrong with this. Maybe it is that I dont understand the problem well, because English is not my first language. Could someone make it clearer for me?

The closed property is only allowed for containers.

Try this:

The bedroom is a room. The box is a closed openable container in the bedroom.

Thank you. Now it sounds obvious.

How do I make empty line?

When play begins: say "This in the best game ever made. Your mission is go to the living room."; say " ".

doesnt work.

Edit: I don´t know what I did wrong, now it works. It is so illogical. Well, main thing is it works.

There’s some special magic that kicks in when you say something that ends with a period, so that you usually don’t need to add extra line breaks add the end. This doesn’t happen if you have a space after the period, so

say "Something."

Will render differently from

Say "Something. "

(see that extra space at the end?)

To insert breaks manually, you can say [line break] or [paragraph break].

You can also use [run paragraph on] to prevent it adding a newline.

Actually, any thing can be closed, you just have to tell Inform it can be. For example, the standard rules define

[code]A door can be open or closed. A door is usually closed.
A door can be openable or unopenable. A door is usually openable.

A container can be open or closed. A container is usually open.
A container can be openable or unopenable. A container is usually unopenable.[/code]
and you can do the same for any other thing or kind of thing that you like.

There’s an example of this in “4.7. New either/or properties” of the Inform Documentation, which shows how to make an umbrella be open or closed.

Hope this helps.