What should I do if I resign after working as a manager in Evergrande's property company for a period of time and the handover is completed, but I am not approved to leave?

You can report the situation to the labor bureau and the labor inspection brigade, collect evidence for what you think is unsatisfactory, and get overtime pay or labor remuneration through labor arbitration. There is no reason not to check out and leave as long as the resignation formalities are submitted one month in advance and the handover is completed. I especially remind you to have hard evidence against this company, and they will take the initiative to find you to solve the problem.