Do you know why I get this error?
I need more than 100 static pages. And I thought that I can use dynamic pages as static pages 
So I just created a dynamic page and wrote my essay to it. There was no problem in the preview but on the real site I get this error :
There can be several reasons
- Entering information only in the sandbox and not in the live
- or the database permissions read only to members.
thanks but How can i check the database permission and also i didnt write anything on the database. Should i?
A dynamic page gets the information from the database, if your live database is empty, this is of course why you come to an empty page because there is no such page.
You need to log into your database, enter information on the live site, or simply move the sandbox to live and then from there to the dynamic page to its address
See here for working with dataset modes and permissions.
https://support.wix.com/en/article/working-with-dataset-modes-and-collection-permissions
Along with syncing your dataset.
https://support.wix.com/en/article/syncing-content-between-sandbox-and-live-collections
Plus, I would suggest that you have a read about dynamic pages.
https://support.wix.com/en/article/about-dynamic-pages-4488730
https://support.wix.com/en/article/dynamic-page-example
What you have probably done is created your dataset and set it to be Admin only, so you will be able to view it easily in your Wix Editor and preview mode as you are running as Admin.
Whereas, when you run the published live site the public user will be classed as public and any site member will be classed as member, so neither of these users will be able to access the page as they do not have the correct permissions.