This is because of your security permissions that do not allow you to delete submitted entries. Your Core administrator can turn that security permission on for you (Allow edit submitted entry under Time Entry on Security > Edit/Customize Profile). Check the Core Help for details.
Note: If your company complies with DCAA, you should not alter the security permissions, but rather un-submit the entry first and then delete it. Check out this article for details: Editing time when Core is set to be DCAA compliant.