How to fix the wall

I didn’t know in which subreddit to ask but since I haven’t seen such thing in my home country, maybe there are more people in Germany who might know what’s up since I have this problem in an apartment here.

I rent an apartment and it had such gaps on the wall since I moved it. It’s not everywhere but I would love to fix it if possible because it’s just ugly. I’m not sure if it’s wallpaper or wallpaper and paint or something else. Could I just paint over it? Of course it won’t be perfect but not so visible at least. Or it would break the wallpaper (if it’s actually a wallpaper). It’s just such a weird way how the walls are done in this apartment. Of course I could cover it with some posters but some wall parts have black lines on it and it would be way too many posters.

When I moved in, it was promised that the walls will be dyed but it didn’t happen and I’m pretty sure the landlord won’t do anything to help so I hope there is something I can do on my own.