I'm using TurboTax Premier 2021 to file federal 1040 for my child. It complains about "Form 8615: ParentsNetLTGain shouldn't be less than or equal to zero." Bug?

That entry comes from my Sch. D. line 15 Net long-term capital gain or (loss), which is negative due to net loss. So it appears to be a TurboTax bug.