Orgs using MUA - multi-user assignment.
- Triggers using the trigger action Add to sequence using Prospect owner's default mailbox configuration.
MUA is currently not designed to work with triggers if there is no primary assignee owner. If you are using MUA and want your triggers to add prospects to sequences, make sure one of the assignees is the primary assignee owner, like below. This can be bulk updated from the prospect list view page, and/or added as a field mapping in the CRM to the Owner field.