I filed a return outside of Turbo Tax but believe I can file an amended return to take advantage of a tax credit for EV charger installation. Can I make that amendment using turbo tax if the original return was not completed using turbo tax?
Ideally you should go back to who did it and they should amend it for free if it was their error. To amend a return you did somewhere else or you don't have the original .tax file, you would first have to fill out Turbo Tax like you are doing the original return and pay for it. You have to recreate the actual wrong original return. Then when it matches each line that you really filed you have to tell Turbo Tax you will mail it so it thinks it was filed. Then you can amend it.
For the current year you can use the online version by first recreating the original return exactly as filed. Then pay the fees and say to mail it and print it out so it think you filed it. Then when it matches every line you can amend. How to amend an Online return
https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-filed/00/25588
Or you can buy the CD/Download Desktop programs here,
https://turbotax.intuit.com/personal-taxes/cd-download