Hi @Hayden Goldman ,
You can control it by using Microsoft 365 admin center.
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As an administrator, how do I approve other users to download custom Outlook add-ins? The add-ins downloaded via a URL? Many thanks!
This is the error message the user is seeing:
Just checking if the information TKujala mentioned above could help solve your issue.
Besides, I also found a PowerShell method, for your reference: Specify the administrators and users who can install and manage add-ins for Outlook.