<p><strong>problem solved by resetting cura.</strong></p>

<p>I have problems like this: How can I fix this? I can't find the right setting.</p>

<p><a href="https://i.stack.imgur.com/2cf9Q.png" rel="nofollow noreferrer" title="No top layer"><img src="https://i.stack.imgur.com/2cf9Q.png" alt="No top layer" title="No top layer"></a></p>

<p>Also, what are these yellow lines? Do you know how to remove them? They disappear when I disable to show the brim/raft/skirt.</p>

<p><a href="https://i.stack.imgur.com/Bj7Sx.png" rel="nofollow noreferrer" title="Yellow lines #1"><img src="https://i.stack.imgur.com/Bj7Sx.png" alt="Yellow lines #1" title="Yellow lines #1"></a></p>

<p><a href="https://i.stack.imgur.com/BnAc4.png" rel="nofollow noreferrer" title="Yellow lines #2"><img src="https://i.stack.imgur.com/BnAc4.png" alt="Yellow lines #2" title="Yellow lines #2"></a></p>

I'm pretty sure the yellow lines are showing the full path of the extruder head, including where it's retracted.  Somewhere in Cura's maze of menus, there's an option to turn on/off various displays related to the slicing.
As to why the top layer isn't there -- most likely it's too thin in your source  model.  You might try enabling "Print thin walls" options in the Preferences advanced list.  BTW, if you post the original STL or CAD file, we might be able to provide more accurate diagnosis.