In your Databricks workspace unity Catalog enable, go to Compute > Cluster Policies, click Create Policy, and name it UC_Policy to set up a Unity Catalog-enabled cluster policy.

Attach the UC policy to your cluster.

The notebook in the child pipeline is using a Linked Service that's compatible with Unity Catalog.

The way the Linked Service is defined in each pipeline should be reviewed.

It should be verified the user identity Managed Identity or Personal Access Token—changes between pipeline levels to provide correct Authentication type and cluster details.

It should also be checked whether the notebook activity is directly associated with a policy or cluster pool, or if that association is lost when the notebook is invoked through the parent pipeline.

Once all references use the same linked service and policy: Run Master pipeline — it should now work successfully.