This could be a number of issues: user may not have ability proper CRUD permissions, the database is not configured to allow access from the IP address you’re trying to connect from, database information is not entered correctly, the server might not be running (if a local db).

This worked for me once or twice. Try going over to the Custom tab in the Configuration window and entering the database information there. Here’s what you’ll need:

NAME | VALUE

SourceDatabaseHost | [YOUR HOST]
SourceDatabaseName | [YOUR DB NAME]
SourceDatabaseUserName | [YOUR USERNAME]
SourceDatabasePassword | [YOUR PASSWORD]

This has worked with MySQL and PostgreSQL. You just have to make sure the database is empty. Mendix will synchronize your projects domain model with the database you are connecting too.

Reference Link: https://docs.mendix.com/refguide/custom-settings#4-3-migration-settings

Try this:

use mysql;

alter user 'root'@'localhost' identified with mysql_native_password by '********';

flush privileges;