Hello, since recently a piece off a wall keeps appearing, that isn't there. I don't know how to get rid of it. I think it's some kind of glitch.
In this file, you can see the photo.

Can somebody help me?

It's called the floor edge, see here:
http://www.modthesims.info/download.php?t=459824

That's as much as I can say, because I have had the shader replacement and default floor edges in my game for so long, I don't even remember how it's supposed to work without.
Have you downloaded something recently that could have caused this? Have you tried removing all your user content (after you've made a back up of your game) to see if your issue still exists without CC?

From what I see in your picture, this is not a wall and not the result of a bug. The edge of the floor/ceiling just looks like that from some camera angles. I know of two options if you don't like the way it looks:

1. Use the mod linked above (there's a link on that page to another page which includes Mootilda's invisible floor texture which you can use with it).

2. Turn ceilings off in your game (the odd shaped button above the family funds display and next to the video camera icon).

Thanks everybody but i found what the problem was i had to turn off the ceilings in the game. This worked for me