The Blizzard Battle.net desktop application uses a process called Agent to install and patch Blizzard games. The steps below may resolve problems in which Agent can’t download data or install files.
If your region is currently patching or undergoing maintenance there might be a known issue affecting all players. Check @BlizzardCSEU_EN on Twitter, or the technical support forums linked at the bottom of this article, for information on any ongoing issues.
Update or temporarily uninstall any security programs, which may mistakenly identify the login module as a security threat.
If you're using a wireless connection, optimize your internet connection to rule out a connection issue.
Update your drivers and operating system to resolve any compatibility issues.
Try creating a new administrator account to resolve permissions issues.
Note: World of Warcraft addon managers can prevent Agent from updating game files. Disable or temporarily uninstall these programs, restart your computer, and try again.