I have a 2018 w2 that i did not file. can i add it to my 2019 return?

NO.  The 2018 W-2 cannot be entered on 2019 tax return.  The 2018 W-2 can only be entered on a 2018 tax return.

No, your 2018 income must go on a 2018 return. If you did not file 2018, you will need to purchase the 2018 program and file. If you did file, you will need to do an amended return to add the W2.

Here is the link to purchase 2018 software. 

Here is the link for amending a return.