For approvers and authorizers to be shown in the dropdowns in a FlexOrg, the user needs:

1. To be assigned to a Role Document with the appropriate permissions under Setup -> Governance -> Automated Actions

2. To be assigned to the FlexOrg where the policy is defined. Assignments are not inherited by sub-FlexOrgs

NOTE: Be sure it is configured at the same level as where the user is defined.

i.e.  a sub-FlexOrg vs. a parent FlexOrg.

For example, as an admin in the top level FlexOrg, you appear as an approver/ authorizer for any policies defined in the top level FlexOrg, but not in any of the sub-FlexOrgs.