Solution:
I also faced above error , in my case it was actually due to the wrong username and password.
Solving the issue:
a. Go to the line DriverManager.getConnection("jdbc:mysql://localhost:3306/?useSSL=false", "username", "password");
b.The fields username and password must be wrong. You must enter the username and password which you use to start your mysql client.
Further Readings:
https://github.com/speedment/speedment/issues/621