When you get to the screen that titled "Enter Prior Year Roth IRA contributions", make sure you are entering the total amount of all your previous Roth contributions. So if you contributed $8,000 to your Roth IRA in prior years, you will put $8,000 in the amount box on this screen.
You can always withdraw contributions (but not earnings) that you made to your Roth IRA tax and penalty free at anytime. Additionally, the Ordering rules for withdrawals from a Roth IRA are: first from regular contributions, then from Conversion and rollover contributions, on a first-in, first-out basis and finally from Earnings on contributions
