Summary

When a project administrator grants additional project roles to users already added to the project (under Project Settings > People) the below error is seen:

Environment

Jira Cloud

Cause

The "Browse Users and Groups" global permission is required to be able to view and select users in any user picker fields. If the administrator granting roles to existing users doesn't have this permission, it will result in the above error.

Solution

The site administrator can grant the "Browse Users and Groups" global permission to the required users to fix this issue. The detailed steps to grant global permissions to a group of users can be found in this article:
Grant global permissions to a group of users