Many companies do requisitions in different ways.
At some companies the people that perform hard-booking of resources are the Resource Managers rather than the Project Managers.
If we look for the permission in order to do hard-booking we have the following access rights.
Resource Management Resource - View - All
Resource - Hard Book All
Resource - Soft Book All
Resource - Update Skills - All
Project - Create/Edit Requisitions - All
Project - Attach Requisitions Entry Resources - All
Project - View Requisitions - All
This info is obtained from the Access Rights Reference Guide, from <https://support.ca.com/cadocs/0/CA%20Clarity%20PPM%2014%202%2000%20%20On%20Premise%20-ENU/Bookshelf_Files/HTML/access/2179061.html>
Now when you add these permissions and try to book a resource, it will show the following error.
As the error indicates show there are missing access rights and this access is:
Project - View Management - All
This permission allows user to view the general and management properties and processes on all Projects which have been enabled for management.
So in addition to the Hard Book rights the resource manager also needs to have access to see project Management information in order to book a resource.