How do I pay tax due to the state of Iowa?

You can pay directly on the Iowa state website, or print a voucher to mail in your payment.
 

Iowa Dept. of Revenue

I can't find where to print the voucher from Turbo Tax. In years past when I completed filing, the site prompted me to print it immediately. This year it didn't prompt me, and now I can't figure out how to find it.

You don't have to print it from TurboTax. You can print the voucher from the Iowa link provided here, or pay directly. 

If you wish to use a generated TurboTax voucher, go to the Print center and save all worksheets for your tax file.