How make a filter "does not contain" in Google Stackdriver logs

just add AND NOT between two rows:

resource.type="container"
resource.labels.cluster_name="mycluster"
textPayload!="Metric stackdriver_sink_successfully_sent_entry_count was not found in the cache."
severity="INFO" AND NOT
textPayload:(helloworld)

You can use the filter-text:unicorn if you want to exclude entries containing the word "unicorn"

Source: https://cloud.google.com/logging/docs/view/basic-filters