I filed a 1040 by mistake, when I intended to file a 1040NR. I see that TurboTax does not support 1040NR, only Sprintax. Will I be able to amend to 1040NR online?

Use Form 1040-X to amend Form 1040-NR. Also use Form 1040-X if you should have filed Form 1040 instead of Form 1040-NR, or vice versa.
 

To amend Form 1040-NR, or to file the correct return, do the following.

1. Enter your name, current address, and social security number (SSN) or individual taxpayer identification number (ITIN) on the front of Form 1040-X.
2. Don’t enter any other information on page 1. Also don’t complete Parts I or II on page 2 of Form 1040-X.
3. Enter in Part III the reason why you are filing Form 1040-X.
4. Complete a new or corrected return (Form 1040, 1040-SR, 1040-NR, etc.).
5. Across the top of the new or corrected return, write “Amended.”
6. Attach the new or corrected return to the back of Form 1040-X.