Id like to file my return but I already set up a payment plan through the IRS and state FTB websites

[BurakG]

You can login your account and follow the steps the file your return. It will tell you your tax liability for the year, if you already have a payment plan that includes your 2018 tax liability you can make the payments to the IRS or FTB as was determined when you applied for your payment plan. 

If you do not file your return you will be subject to "failure to file penalty" in addition to your regular tax liabilities.