Rollback Script for Purview Auto Labels Using PnP/Graph – Anyone Done This?

Hello and thanks for reaching out. 
I would expect this error because some Microsoft Graph APIs are metered. Label assignment/removal for SharePoint and OneDrive via Graph uses protected (metered) APIs such as assignSensitivityLabel, which require enabling metered APIs and linking the app to an Azure subscription (PAYG). This is explicitly a paid, premium capability. Here is an overview of metered APIs
Assign Sensitivity Label is mentioned as a Metered API