Symptoms

You've enabled credentialless discovery but you are getting an error on the pattern: 

In payload invalid data source [CredentiallessDiscovery] exist. You need to provide a valid choice value from field [discovery_source] in table [cmdb_ci] 

Cause

You are missing the CredentiallessDiscovery choice from the Discovery Source column on cmdb_ci

Resolution

Manually create or use the attached record.