Please contact TurboTax Customer Support during business hours. A Support agent should send you a download, without charge, of the 2018 program you purchased. Since TurboTax Online for 2018 is now closed, you'll have to use the desktop software. You can finish your return that you started in the online platform by following the instructions included in the TurboTax FAQ linked below.
To contact Support, see the instructions linked below.
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Additional things you should know: You'll be required to print and mail your tax return when completed, as TurboTax 2018 e-filing is also closed. Don't forget to sign your paper return, and mail using certified mail or some other service that provides proof of delivery to the IRS.
And....you have an extension to FILE your tax return, but not an extension to PAY any taxes that you may owe. Be sure to go ahead and make payment to the IRS and state(s) if you believe you'll owe taxes in order to avoid interest charges.