How to fix it? Please help with detail explanation!
Here is a detailed explanation of your problem in the Magento DevDocs:
could you please explain to me what to do in steps? I have the same problem and don't know much about this.
It could be any number of things, directory permissions, SELinux permissions, web server configuration error etc. The first place to look is in your web server error log to see if it has anything to say. Failing that describing your set-up such as what operating system, web server you are using etc might help with giving the answer.