Update the field-level security settings for the Primary Resource field on the Assigned Resource object for user profiles that need to access multi-resource scheduling.
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From Setup, open Object Manager.
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Click Assigned Resource to open it.
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Click Fields & Relationships, and then select Primary Resource.
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Click Set Field-Level Security.
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Select the Visible checkbox for all profiles that need access to multi-resource scheduling in Salesforce Scheduler, including the System Administrator.
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Save your changes.