Any hint on how to solve it?

Hi Zhiguo, this means that your runs were scheduled but never began running after 24 hours. Most likely this is because you didn’t start an Agent with the appropriate labels; see here for more info on agents: https://docs.prefect.io/orchestration/agents/overview.html

It's solved. The flow's tags didn't match the agent's tags and this caused the issue. Thank you, Chris.