I filed my 2020 on turbo tax can i go back and add a w2?

You cannot change or add anything on the return that you have already  e-filed.  It is too late, just like when you put an envelope in a US mailbox it cannot be taken back. 

Now you have to wait until the IRS either rejects or accepts your return. Which won't be until Feb 12 this year. If your return is rejected, you will be able to go into your account and add or change it and resend it.

If the IRS accepts your return, then you have to wait until it has been fully processed and you have received your refund or paid the tax due. THEN you can prepare an amended tax return 1040X  but in the meanwhile, DO NOT change anything on your return until you start the amended return. If you did change or add the missing info then take it out and put your return back to the way it was when you first filed it.

You can check your efiling status here,
https://turbotax.intuit.com/tax-tools/efile-status-lookup/

How to amend an Online return
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577

If you have already filed your return, you need to wait and see whether your return gets accepted or rejected. The IRS currently won't start accepting or rejecting returns until February 12, 2021.

• If your return is rejected by the IRS, then all you have to do is make the changes on your return and resubmit the efile.

• If your return will be accepted by the IRS, then you have to file an amended return (1040-X) to make changes. But you might want to wait until your refund or payment is fully processed.  In the meantime, do NOT make any changes to your original filing.  Also be aware that Form 1040-X can take several months to process.