1. Wrong username or password: Please make sure you are using the correct username and password, and open the yml configuration file or the properties configuration file. 
2. Insufficient privileges: Please ensure that the user has sufficient privileges to connect to the MySQL database.
3. The MySQL service is not started: Please make sure the MySQL service is running.
Right-click this computer, manage, service and application, find the MySQL service, and check whether the status is running.
4. Incorrect database connection string: Please ensure that the database connection string used contains the correct hostname, port number, and database name.