All of my previous tax returns prior to 2018 I can download the return and view my W2. When I download my 2018 return it does not show me my W2, why not?

Actually Turbo Tax doesn't have a copy of your W2.  Just the worksheets you filled out from it.

you need to print all the worksheets.

Try printing from the Print Center

Log back in your TurboTax account and "add a state" to open the return back up (you are not really adding a state, just to open it back up), then

Click on Tax Tools on the left side of the online program screen. 

Then click on Print Center. 

Then click on Print, save or preview this year's return.  Choose the option Include government and TurboTax worksheets

How to save the PDF

https://ttlc.intuit.com/community/saving/help/how-do-i-save-my-turbotax-online-return-as-a-pdf/00/26234

I NEED A COPY OF MY W-2

TurboTax does not have a copy of your W-2—the Wage & Tax Statement—issued to you by your employer, (nor does TurboTax have copies of your 1099’s).   You should always save those documents for your own records. You could ask the employer(s) for copies or you can get a copy from Social Security (for a fee of $86).

If you saved a pdf of your completed tax return, the worksheets will show the W-2 information you entered, but not the actual W-2.

To get a copy of a W-2 from Social Security https://faq.ssa.gov/en-us/Topic/article/KA-02501

https://www.irs.gov/help-resources/tools-faqs/faqs-for-individuals/frequently-asked-tax-questions-answers/irs-procedures/transcript-or-copy-of-form-w-2/transcript-or-copy-of-form-w-2

You can get a wage and income transcript from the IRS:  It will not show the actual W-2, 1099’s, etc, but will show the income information the IRS received.  It will not, however, show your state information. It will only show the federal information.

https://www.irs.gov/individuals/get-transcript

https://ttlc.intuit.com/questions/2561916-how-can-i-get-a-copy-of-my-w-2