How can I file only the state taxes, not federal?

You cannot e-file a state return unless the federal return has been e-filed and accepted.  If you are worried about owing tax for the federal--remember you have until July 15, 2020 to pay that.  You are allowed to go ahead and e-file the federal now but say you will pay by check.  You would still have until July 15 to pay, and you would be able to e-file your state.

By July 15, 2020---

If you have federal tax due you can pay by mailing your payment with the 1040V voucher, having the payment taken out of a designated bank account, or you can pay directly on the IRS website.

https://www.irs.gov/payments/direct-pay