If you have previously e-filed you should use the same PIN again. If you do not remember your PIN, you can use your last year's AGI in such case. You should enter the AGI from your last federal tax return on question "Please enter the AGI from your last processed federal tax return" and you will be able to proceed further.
Info: If this is your first time e-filing, please create a 5-digit PIN and make sure to remember it.