For amending your returns, we advise you to wait until you receive your refunds to ensure that the original returns have been completely processed. Trying to amend prior to your original return being completely processed will likely confuse and complicate things at the tax agencies and may unnecessarily delay processing of the amended return.
The Form 1040-X for the amended return only recently became available in TurboTax, which might explain the rejection that you saw. Don't try to file the return again until you are sure that your original return has already been processed.
This link has helpful tips on amending returns.
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