I have not mailed or paid my federal taxes from 2018. How am I able to download and print a copy so that I can mail the form with payment?

You have to pay for the TurboTax account before you can either e-file or print the tax return.  In the File section of the program on Step 1 pay the TurboTax account fees by credit card.  In Step 2 of the File section you are given several options on how to pay the taxes owed.  In Step 3 of the File section you are given the option to either e-file the tax return or to print the tax return for mailing.

See this TurboTax support FAQ for the procedure to print a tax return for mailing when using the online editions - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online/01/26258

You can also efile your return until Oct 15.  Is there a reason you can't efile?  You can still mail a check for tax due.

How to Efile

https://ttlc.intuit.com/community/e-file/help/how-do-i-e-file-my-personal-individual-tax-return/00/25891