I have already filed my taxes but n the process forgot to file my 1099-R

[DoninGA]

If the return is accepted you will have to amend your original tax return. An amended return, Form 1040-X, can only be printed and mailed to the IRS if the original tax return was not e-filed. The IRS will take up to 20 weeks or longer to process an amended tax return.

Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.

See this TurboTax support FAQ for amending a current year tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-curr...

[JamesG1]

If your return has been transmitted and rejected by the IRS, you can correct your tax return and re-transmit. 

If your return has been transmitted and accepted by the IRS, you will have to amend your Federal 1040 tax return.  However, before amending your tax return, do not enter the return and do not make any changes to the return.

I would wait until you receive the refund from the original income tax return, to make sure that the IRS agrees with the numbers that you submitted.

If you used TurboTax Online, log in to your account and select Amend a return that was filed and accepted. 

If you used the CD/download product, sign back into your return and select Amend a filed return. 

See also this TurboTax Help.