Project and project task in multiple business unit can't be used on actual transactions

Summary:

Why I only can create a PR (Purchase Request) using a business unit SCS-BU for that project? and cannot use other business units based on the organization that I have assigned to each project task.

Content (please ensure you mask any confidential information):

I have created an organization trees with legal employer as project and task owning with several business units below it

and I assigned that legal employer as organization under business unit SCS-BU

Currently at Financial Project Plan under business unit SCS-BU the organization belongs to related legal employer

so I can assign the legal employer and the business units related to the organization trees to every project task level