Enable the use of custom sensitive information types in AIP
Documentation page https://docs.microsoft.com/en-us/azure/information-protection/configure-policy-classification states that any custom sensitive information types that have been defined in Office 365 cannot be used in configuring AIP Labels. Please enable the possibility to use custom sensitive information types as well as the Microsoft defined sensitive informantion types.
This functionality is supported as part of AIP Unified Labeling client. More information is available here: https://docs.microsoft.com/en-us/Office365/SecurityCompliance/apply_sensitivity_label_automatically