Hi @john john ,
ClientSideAssets is a special library which by default allow all internal user access deployed Js files in browser even there is no permission settings in the UI.
While for external user, the library won't provide any permission even for js file in browser, it's necessary to set at least read permission for all external user.
To grant permissions to all users including all external users at once, we can do this using SharePoint Online Management Shell by following below:
Connect-SPOService
Set-SPOTenant -ShowAllUsersClaim $true
Then grant permission to the group named "All Users (membership)":
Referene:
Permissions Required For External Users To Use Modern UI (SPFx) Components
Grant permissions to all external users on Client Side Assets library in App Catalog site
Thanks
Best Regards
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