After you initially start Sniper Elite 3, particularly for the Ultimate Edition which comes with additional DLC packs, you may receive a message that says:
This portion of the title has not been installed yet.
Please try again when installation is complete.
Sniper Elite 3 downloads and installs DLC packs in the background while the game is running, and this process can take up to an hour or more, based on some internal tests we have run in live, retail environments.
Do not uninstall and reinstall the game! This will just reset the whole process, and require the game to install all of the additional content from the beginning again.
This will not stop you from starting a campaign with the base content. However, we encourage you to wait until all content is installed and you can access all that the Ultimate Edition of Sniper Elite 3 has to offer!