Some additional content is needed for this concert. Do you want to download it now?

You are opening MainStage, and get a popup window that says "Some additional content is needed for this concert. Do you want to download it now?"

This is a common message that shows in MainStage if you've opened a concert file that requires additional content from the Apple Sound Library to be downloaded. If you see this message when opening products such as Worship Essentials, then make sure to download and install everything you are prompted to do.

In very rare cases, MainStage may exhibit a bug that will cause the additional content popup to show every time the concert file is opened, even if you press "Download". In that case, you will need to open the Sound Library Manager (Menu location: MainStage > Sound Library > Open Sound Library Manager...) and manually check the boxes and install the sounds from there. That should cause the popup window to stop showing.

If nothing seems to work and you continue to get the popup window, try reinstalling the Sound Library: (Menu location: MainStage > Sound Library > Reinstall Sound Library...)

Further reading about the Apple Sound Library: https://support.apple.com/guide/logicpro/manage-logic-pro-content-lgcpf77e757d/mac