Hi Medo,
Welcome to our community! 
In which course and lecture are you?
Here are a few suggestions:
- Save your build on your harddrive but not inside your project folder, on your Desktop or in a cloud.
- Use English characters (A-Z, a-z, 0-9) only for your file and folder names.
- Remove all scenes from the Build Settings list and readd them.
- Make sure that neither your firewall nor your antivirus program block Unity.
- Run Unity with administrator rights.
- Update Unity to a newer stable version.
Hopefully, one of them helped.
See also:
- Forum User Guides : How to mark a topic as solved
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