Hi @Gavin ,
Does this issue happen to all users?
1.Please check the external sharing settings for your organization.
Microsoft 365 admin center > SharePoint admin center > Policies > Sharing > select Anyone sharing level for SharePoint and OneDrive.
2.Please check the external sharing settings for the site(both SharePoint and OneDrive).
SharePoint: Microsoft 365 admin center > SharePoint admin center > select your site > Sharing > Select "Anyone"
OneDrive: Microsoft 365 admin center > Users > Active users > click user > OneDrive > Manage external sharing > turn on "Let people outside your organization access your site" and select "Allow sharing with anonymous guest links and authenticated users"
3.Please check Limited-access user permission lockdown mode
Settings > Site information > View all site settings > Site collection features > Deactive "Limited-access user permission lockdown mode" feature
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