After creating the databricks_metastore resource, can you run databricks_grants? like this 

resource "databricks_grants" "foo" {
depends_on = databricks_metastore.foo
  metastore = databricks_metastore.foo.id
  grant {
    principal  = <your service principal>
    privileges =  ["CREATE_EXTERNAL_LOCATION", <other priviliges>]
  }
}