Unable to assign bank account to supplier site level - access invalid

Summary:

We are trying to create employees as suppliers and assign bank account to the supplier using REST API (/fscmRestApi/resources/11.13.18.05/instrumentAssignments) via OIC integration. We are getting Access invalid error even though the integration has supplier manager and supplier administrator roles.Is the integration user required to an employee/procurement agent to assign bank accounts to suppliers at site level or is there any way to get through it without making the user and employee or contigent worker?

Content (required):

We are trying to create employees as suppliers and assign bank account to the supplier using REST API (/fscmRestApi/resources/11.13.18.05/instrumentAssignments) via OIC integration. We are getting Access invalid error even though the integration has supplier manager and supplier administrator roles.Is the integration user required to an employee/procurement agent to assign bank accounts to suppliers at site level or is there any way to get through it without making the user and employee or contigent worker?