You would not do either an extension or an amended return for your problem.
The filing deadline for 2018 was a year ago--you cannot get an extension for 2018. The last date you could request an extension for a 2018 return was April 15, 2019--which would have given you until October 15, 2019 to file that return.
If you are getting a refund, there is not a penalty for filing past the deadline. If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.
Amending is a special process to use for a return that has been fully processed--you say the IRS did not get your return.
You do not say when you mailed the 2018 return. Was it over a year ago or was it sometime in 2020?
If you mailed 2018 in 2020:
The IRS has just started to work through millions of mailed returns that have been piling up in trailers during the pandemic.
Per the IRS website IRS Operations During COVID-19: Mission-critical functions continue:
IRS operational status and alternatives
Processing Delays for Paper Tax Returns: Taxpayers should file electronically through their tax preparer, tax software provider, or IRS Free File.
We’re experiencing delays in processing paper tax returns due to limited staffing. If you already filed a paper return, we will process it in the order we received it.
Do not file a second tax return or contact the IRS about the status of your return.
If you mail a tax return (or a payment) to the IRS, it is a good idea to use a mailing service that will track it like UPS or certified mail so you will know it was received.
If you mailed the 2018 return last year--sometime in 2019 and it was not received --- then you can print and mail that same return again. Do NOT click on anything that says "amend." And if you mail it again, use a mailing service such as certified mail so you will know it was received. It will be a long time before the IRS processes it.
When did you mail them? If you mailed 2018 this year the IRS might not have opened your envelope yet because of the Covid-19. They stopped opening the mail in March and now have millions of returns to open. Or did you send both federal and state returns in the same envelope? Then one got missed. They go to different places.
If your first return never got filed you do NOT amend. Just mail it again now.