@sampoyigi and support team!
For the understating purpose, I tested your code at order with combination of 1 coupon Fixed Amount of 121 and item worth 100.
Strangely, order placed success using COD payment worth -21
. See below cart detail:
What you've ordered:
1 x Margherita
Large = ₹120
₹100 ₹100
Sub Total ₹100
Coupon 110OFF ₹-121
Order Total ₹-21
We hope to see you again soon
I think, you should have to check positive order amount only be place at coupon apply time as well as check out action time.
Hope this will help you to sharp your project!
Thanks!