You want to create a DLP Policy for G Suite Securlet for Searchable setting but you do not have DLP Enforce integration.
CloudSOC, Cloud DLP
No DLP Enforce server available for integration.
Please note: For full functionality use DLP Enforce integration in order to use a contextual attribute for Searchable "common.doc.searchable" and other attributes (see Configuring DLP policy to use the searchable option in the scan filter)
Create a DLP Profile which matches on Envelope and looks for the below keyword:
common.doc.searchable: true