Followed the instructions.
It seems the role "Notifications Manager" needs to be defined first.
% ibmcloud iam authorization-policy-create functions cloud-object-storage "Notifications Manager" --source-service-instance-name <source-functions-namespace> --target-service-instance-name "target Cloud Object Storage service with spaces"
Creating authorization policy under account ...'s Account as ...
FAILED
Role Notifications Manager was not found. Valid roles are Writer, Reader, Manager, Content Reader, Administrator, Operator, Viewer, Editor.
Followed the instructions.
It seems the role "Notifications Manager" needs to be defined first.