Hello, I was wondering if there was a way to prevent a scheduled flow that is scheduled to run every 30 mins, to not start if the previous run has not finished. Any ideas how to do that?

Yes! You can do it using a flow-level state handler:

import prefect
from prefect import task, Flow
from prefect.client import Client
from prefect.engine.state import Skipped
import time


@task(log_stdout=True)
def hello_world():
    print("Sleeping...")
    time.sleep(360)


def skip_if_running_handler(obj, old_state, new_state):
    if new_state.is_running():
        client = Client()
        query = """
            query($flow_id: uuid) {
              flow_run(
                where: {_and: [{flow_id: {_eq: $flow_id}},
                {state: {_eq: "Running"}}]}
                limit: 1
                offset: 1
              ) {
                name
                state
                start_time
              }
            }
        """
        response = client.graphql(
            query=query, variables=dict(flow_id=prefect.context.flow_id)
        )
        active_flow_runs = response["data"]["flow_run"]
        if active_flow_runs:
            logger = prefect.context.get("logger")
            message = "Skipping this flow run since there are already some flow runs in progress"
            <http://logger.info|logger.info>(message)
            return Skipped(message)
    return new_state


with Flow("skip_if_running", state_handlers=[skip_if_running_handler]) as flow:
    hello_task = hello_world()

That is excellent, thank you so mcuh