A resource manager cannot add Resource Manager's resources into the project manager's (PMs) project
View access to the project is required at a minimum in addition to the Resource Manager (Auto) rights. See Add / Book team members - access required for more details.
Release : All Supported
Component : Clarity Resource Management
Grant the resource manager project view access to the project (This can be done at the instance, OBS, or Global level, and can be done for the specific resource or within a group to make the change for all users).
Make sure the user logs in and logs back out for rights to refresh after the access is granted.
See also:
For the Modern UX, see: Unable to add staff to project in New UX