You apparently chose to pay your TurboTax fees out of the Federal refund, and your Federal refund must not have been forthcoming yet at this point in time or was reduced by the IRS. When TurboTax sends out a billing email, here are a couple of possibilities that can trigger that:
1) Either the IRS reduced the amount of your Federal refund such that it would no longer cover the TurboTax fees,
OR
2) Your refund has simply been delayed at the IRS. If it's taking the IRS a long time to process your refund--for whatever reason--TurboTax will want payment if it goes 36 days past your original estimated refund date.
You can check the status at the IRS "Where's My Refund" site to see if the IRS has released the refund, or whether it's delayed, and if they are still processing it.
https://www.irs.gov/Refunds
Pay attention to the billing emails you receive from TurboTax. If you don't pay or contact Customer Support to request a deferment, they will eventually send you an email saying that they are about to debit your bank account. If you still don't respond, then they will debit your bank account.
If you can't pay it right now, contact Customer Support and request a deferment so they won't debit your bank account.
FAQ: What is the TurboTax phone number?
https://ttlc.intuit.com/questions/1899263
