On the Modern UX in Clarity I can not find the plus sign (+) to add resources to investments, nor investments to resources
What does control the ability to add?
If Mixed Bookings are enabled, the Staffing pages are read-only
Release: Clarity 15.9.1 and lower
As per Leverage Mixed Bookings in the Staff Grid - Editing Allocations When Mixed Booking is Enabled section:
When mixed bookings is enabled, the Staffing page is available in read-only mode. Hence, you need to perform staffing activities in individual projects.
In order to disable mixed bookings, you must convert all your existing Mixed Bookings into either Soft or Hard. You can check this by looking at the Booking Status portlet in the Classic UI Resource planning portlets
Alternatively query the DB:
select * from prteam where prbooking = 10
In 15.9.2 the MUX supports Mixed Bookings fully and the Staffing section is editable when Mixed booking is enabled.