Can you do amended tax return if I was not using turbotax to file my tax return?

You can but there are things you need to do before you amend.  You would need to first enter all your information exactly as it was when you filed your original return.  Then you need to "trick" the program into thinking you filed by choosing to file by mail.  If you can't use the Free Edition you would have to pay before you can print your return which you have to do to trick the program into thinking you filed..  It might be easier to go back to where you filed your original return.

After all your original information is entered & you "tricked" the program by choosing file by mail, save your return, then you can go through the amendment interview to amend your return.  Amended returns must be mailed.

To add:  You can only use the Online version for a 2018 return.  If you are amending a prior year return, you need to purchase the desktop software to amend.  You still follow the steps above.
<a rel="nofollow" target="_blank" href="https://turbotax.intuit.com/personal-taxes/past-years-products/">https://turbotax.intuit.com/personal-taxes/past-years-products/</a>