Hi MahmoudOth,
I think it was becuase you had renamed a copy of the .aspx file which still using the newer versions code behind file.
The code behind file was looking for Literal control with id ltEmbed but on the older version of the .aspx file ltEmbed didnt exist.
So exclude the older version of the page from the project and run.
Else
1. Delete and add the literal control again.
2. Build the project.
If above does not work then your project is corrupted.
Hope this will help you.