When working in an out-of-box Event Registrant Batch, you may encounter the following error when you use the constituent ID in the Registrant field:
No results for field "Registrant" using search list "Constituent Search" were found. ''Registrant'' is required.
Instead of using the out-of-box Event Registrant Batch, create a custom Batch Template based off the Event Registrant Batch. Be sure to include the field Registrant ID in the Batch Template. When mapping your Import file, map the constituent ID to the Registrant field and leave the Registrant ID field blank. The file will import correctly.