An amended 2018 tax return would have no bearing on filing 2019 tax return or requesting an extension for 2019. A 2019 tax return and/or extension can be filed at any time without regard to the 2018 tax return.
When you say your 2018 return was rejected, are you referring to the return when you filed it sometime before mid October, 2019. A 2018 return could not be efiled any later than mid October 2018. Did you use the 2018 tax program to prepare and file it? You can't use the current online program for a 2018 tax return. It is only for preparing a 2019 tax return.
