fanfare is correct. TurboTax doesn't know what to do when you're trying to pay less than what's due right now. The only way I know of is to tell TT that you'll pay by check, and then send your payment manually not through the software.
A warning though, technically you're not in compliance when you follow a self-imposed payment plan. The IRS may charge you late fees and interest. If you prefer to follow the rulebook, especially if you can't pay this off quickly, you'd need to tell the IRS you need an installment agreement. There's a bunch of info about it on the IRS.gov website.
