What do you mean by that, exactly?

See Reporting Problems.

We need you to say exactly what you're doing and what's happening — see the linked page.

I understand that, but if you want help here, we need you to provide information in the form we ask for.

Specifically, where are you in the software when you're seeing this? What exactly are you doing before the message appears? We need you to tell us literally what you're clicking on on the screen when this happens.

Specifically, where are you in the software when you're seeing this? What exactly are you doing before the message appears? We need you to tell us literally what you're clicking on on the screen when this happens.

after I enter my username-password, its promting invalid username or password

No, it's not. If you simply enter your username and password, nothing happens. So what are you doing between when you type your password and when you get the error message? Are you clicking the Set Up Syncing button, or are you clicking OK to close the preferences window and then clicking the Sync button? The problem will be different depending on which one it is, which is why we need you to tell us exactly the steps you're taking — as the linked page says, that includes every single thing you're clicking on on the screen.

If you're clicking Set Up Syncing, you've simply entered your username or password incorrectly. You should clear both fields completely and try again.

If you're clicking OK to close the dialog, you haven't actually set up syncing, and you need to click the Set Up Syncing button first.