Since you have not yet mailed the return, you can make changes to add the missing Form 1099-R without going through the amending process.
In order to get back into your return to make changes, you will need to indicate that you are trying to Add a State. This will open up the return again so that you can edit it.
The step by step instructions are in the following TurboTax article, just stop after step 4 and make your changes:
How do I change from mail to e-file in TurboTax Online?
@Tomographic
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