I mailed my tax return for 2018 on July 30, 2020. Now IRS says they do not have them. How can I get another copy of my 2018 forms from Turbo tax?

If you mailed it in July 2020 it might still be in a pile of unopened mail.  Mail piled up in trailers during the pandemic and the IRS is working on opening millions of pieces of mail.  Did you actually talk to the IRS?  When you mailed the return did you use a service that would track it--like certified mail--so you would know it got there?

 

Here is what they say about mailing a return AGAIN if you already mailed it.

 

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.

Did you use Turbo Tax?  How did you prepare 2018?  If you did them after Oct 2019 then you had to use the Desktop program and the only copy would be on your computer.