Hey Jann,
What exactly do you want to get rid of?
Just by looking at your screenshot, I'd say, it's probably best to just get rid of the whole model and start over; it looks like there is quite a lot wrong with your 'Cube'...
If you want, you can upload your .blend file to Googledrive, Dropbox, or so and post a link here, then we can have a look.
Hello Jann. I agree with Martin, the model looks like has a lot of bad topology and mistakes that got piled up along the way. Don't worry, it's perfectly normal, we all have to start over at the beginning of our Blender journey and even after you're an expert. It is much better to start from scratch than to try and fix it all, you learn much more, your brain retains a lot of the knowledge and you do things much faster and have much less brain fog and things start to click and you suddenly get it.
Thank you for the reply. I solved the problem. It wasn't in Auto shade smooth. So I shaded it flat and applied "auto shade smooth".
