Can Iuse Turbotax to complete an amended Federal return for tax year 2018, even though I did use Tubotax to file the 2018 return? I understand I can't e-file 1040x.

I assume you mean you didn't use Turbo Tax?   Yes but it will be a hassle and you will have to pay for Online or buy the Desktop program.  It will be easier to do in the Desktop program.

To amend a return you did somewhere else, 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.