Can i use the license code for my store bought Deluxe to file an amend to a return filed free online?

If you used the Desktop program to do the original return why do you want to amend it online?  You can't amend online if you did the original return on your computer.  And no, you can't apply the License Code to the online version.

in addition, all amended returns must be paper filed. 

It sounds like you filed using the Free Edition?  Why didn't you use the Desktop Deluxe you bought?  Can't you install it on your computer?  

Do you need to upgrade to Deluxe online to amend?  You can download the .tax2019 file from online and open it in your Desktop program to amend it. We can help you install it and download your return.

If you used the Online version did you try getting a refund for the Desktop program?