Hi C_Murp,
We appreciate you reaching out to us. We understand that you are getting a prompt that says "The Apple Music Apps Content Installer...GarageBand wants to install your download of instruments and sounds. The Apple Music Apps Content Installer is trying to install Apple software." when trying to download new instruments for GarageBand.
To help us isolate the issue, we'd like you to try some steps.
First, we recommend that you check if the issue persists when signed in on a different user account. The steps below will show you how:
"1. On your Mac, choose Apple menu 
> System Preferences, then click Users & Groups 
.
If the lock at the bottom left is locked 
, click it to unlock the preference pane.
2. Click the Add button 
below the list of users.
3. Click the New Account pop-up menu, then choose a type of user.
- Administrator: An administrator can add and manage other users, install apps, and change settings. The new user you create when you first set up your Mac is an administrator. Your Mac can have multiple administrators. You can create new ones, and convert standard users to administrators. Don’t set up automatic login for an administrator. If you do, someone could simply restart your Mac and gain access with administrator privileges. To keep your Mac secure, don’t share administrator names and passwords.
- Standard: Standard users are set up by an administrator. Standard users can install apps and change their own settings, but can’t add other users or change other users’ settings.
- Sharing Only: Sharing-only users can access shared files remotely, but can’t log in to or change settings on the computer. To give the user permission to access your shared files or screen, you may need to change settings in the File Sharing, Screen Sharing, or Remote Management pane of Sharing preferences. See Set up file sharing and Share the screen of another Mac.
For more information about the options for each type of user, click the Help button in the lower-left corner of the dialog.
4. Enter a full name for the new user. An account name is generated automatically. To use a different account name, enter it now—you can’t change it later.
5. Enter a password for the user, then enter it again to verify. Enter a password hint to help the user remember their password.
6. Click Create User.
7. Depending on the type of user you create, you can also do any of the following:
- For an administrator, select “Allow user to administer this computer.”
- For an administrator, select “Allow user to reset a password using Apple ID.”
- Use Sharing preferences to specify whether the user can share your files and share your screen."
The article provides more information about user accounts: Set up users, guests, and groups on Mac
If you are experiencing the issue when signed in on a different user account, we recommend that you check if the issue persists while on safe mode.
Below is an article to help guide you through the process.
How to use safe mode on your Mac
Please keep us posted.
Take care!
