You want to have write access to the Business Object (BO) Identity, but this BO is not PSM released.
SAP Business ByDesign.
SAP Cloud for Customer.
The Identity business object cannot be released for PSM write processes due to the nature of the BO (it is an infrastructure BO, which is not in the application layer control, but in the underlying NetWeaver platform control) and its underlying infrastructural dependencies.
In case you have specific needs, raise this as a requirement, based on which the Identity and Access Management team can explore alternative possible solutions to enable them and plan the required development in future releases.
KBA , SRD-CC-IAM , Identity & Access Management , Product Enhancement