Web services in forms must run under an account. This account is specified by the solution designer (the user building forms and workflows in Composer). Whichever account is specified must be added to a Foundation SAP Policy to be allowed to run against SAP systems. Typically, this is granted through membership in the Script Runners or System Runners roles on the Foundation site.
However, in some cases the account will have those roles and those roles be on the policies but you still receive this error.
Workflow, Forms, User Governance
If the account specified to run the web services are members of a role that is assigned to a policy and you still receive this error, then add the user specifically to the policy.
Working with Policies
This is a known issue slated to be fixed in a future version.