I printed my 2018 return but I want to e-file, how do I do this?

You cannot e-file any kind of 2018 return.  A 2018 return has to be printed, signed and mailed.  E-filing for 2018 has been permanently closed since mid-October of 2019.    And what do you mean you printed the 2018 return but you want to e-file?  Did you prepare a Form 1040X to amend your 2018 return?  You cannot use online software for 2018.  Online is only for 2019--no other year.  It is unclear from what you have said about your 2018 return whether you ever filed it to begin with.  Was your 2018 return fully processed by the IRS?  You do not amend a return that has not been processed.

Amending your 2018 return should not be keeping you from e-filing your 2019 return.  If the 2018 AGI on the tax return does not let you e-file 2019, then try using zero.  If zero will not work then you do have to print, sign and mail 2019.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.