You generally cannot select DBR version in Lakeflow / Spark Declarative Pipelines because the pipeline runtime is Databricks-managed and versionless. Databricks auto-upgrades the runtime for pipelines.

The settings has a JSON view, and some advanced options are only available in JSON.  And the supported control you get is the channel field (current or preview), not an arbitrary spark_version like on Jobs clusters like below:

current = the normal production channel. It uses the current pipeline runtime version, is the default, and Databricks recommends it for production workloads.

preview = the next/upcoming runtime channel. It’s meant for testing your pipeline against upcoming runtime changes before they roll into current.

As of the current release notes, the channels map roughly to:
current → DBR 17.3
preview → DBR 18