DLT materialized views have different properties than regular Delta tables, and vector search indexing may not be directly supported on them through the UI.
Try creating programatically.
client = VectorSearchClient()
# Create the index
client.create_delta_sync_index(
endpoint_name="your-endpoint-name",
index_name="your-index-name",
source_table_name="your.dlt.materialized_view",
pipeline_type="TRIGGERED", # or "CONTINUOUS"
primary_key="id",
embedding_source_column="text_column",
embedding_model_endpoint_name="your-embedding-endpoint"
)