After attempting to e-file and getting rejected, I need to switch to file by mail. Will Turbotax still process the auto payment I authorized? Or should I mail in a check?

If you scheduled a payment, that should still be valid, but I would double check.

IRS Individual Online Account  

Taxpayers with a Social Security number or an Individual Taxpayer Identification Number can create or access their IRS Individual Online Account to get information they need to file their return.  

With an IRS Individual Online Account, people can: 

• View balance owed, payment history and schedule payments. 
• Cancel scheduled payments – new feature for 2024. 
• Get transcripts. 
• View or create payment plans. 
• See digital copies of some IRS notices. 
• View key data from their most recently filed tax return, including adjusted gross income. 
• Validate bank accounts and save multiple accounts, eliminating the need to re-enter bank account information every time they make a payment. 
• View, approve and electronically sign power of attorney and tax information authorizations from their tax professional.