Answer by David McNeill for How to delete or empty a table that is in BigQuery using Python

You can't truncate directly.

The --replace parameter to a bq cli load job has the effect you want, clearing out the existing data. Also referred to as WRITE_TRUNCATE when using API mode.

bq load --autodetect --replace --source_format=NEWLINE_DELIMITED_JSON mydataset.mytable gs://mybucket/mydata.json