This occurs usually in a trial when using a NetSuite Sandbox that is refreshed.
Zendesk requestors emails were previously synced to NetSuite.
Each of these requesters was created by SyncApps in NetSuite and their external IDs (Customer or Company ID) updated in Zendesk for optimization. Since you're using a NetSuite Sandbox and refreshed it, the Customers that were created are now non-existent in NetSuite.
So on the next sync after the refresh, SyncApps is trying to attach the external IDs on the tickets which are now non-existent in NetSuite which triggers the errors.
To overcome this issue please reach out to firstname.lastname@example.org for us to enable a customization to ignore the external ID when syncing. "Do not use External ID for user sync." under Zendesk Customizations.