How do I file a CA 540NR on Turbo Tax for 2021. I received a state income balance due notice from CA but I’m military and my home of record is MI.

Call the California Franchise Board and explain why you were not a California resident. 

That happened to my niece & military husband...when posted in CA.

Living on-base, but they had purchased a car there....and CA is scraping everywhere for potential people who might have sought to evade CA taxation...even when not proper for their situation.

Forgot what they had to do, but, as noted above, calling FTB would be the first step to see if they want proof in some way.

If you had California-sourced income in 2021, then you could have incurred a California income tax obligation.  (Your military pay is not California-sourced.)

You cannot use TT Online to file a previous year's return.  Instead, you must use the desktop version, which you can purchase here:

https://turbotax.intuit.com/personal-taxes/past-years-products/2021/