You need to define the alternate key for the Technicians table by using the TechCheck...

TechCheck ref must be configured as Required before it is used as an alternate key. An alternate key is intended to identify a Dataverse row by a stable business value supplied by an external system. Allowing the key column to be empty would defeat the onboarding integration’s requirement that every technician be addressable by the external reference. The case also states that the same TechCheck reference cannot be used on more than one Technician row; the alternate key enforces uniqueness through its Dataverse index after the key is activated. Searchable controls whether the column is available in search experiences, but it does not guarantee that a value is supplied. Column security governs who can read or update a value, and auditing records changes; neither property establishes the required identity. A sound implementation also validates existing data for nulls and duplicates before activating the key, because key creation can fail while invalid rows remain. This is covered by the Study Guide objectives “configure table properties” and “create and modify columns.” Required is therefore the setting that supports a complete, consistently populated external business key.

Study Guide reference/topic: “Build data models — Configure table properties.” AB-410 Study Guide | Microsoft Learn technical reference