3rd time. Must file 2021 form 8606,not filed in 2020. How?

We'll automatically generate and fill out Form 8606 (Nondeductible IRAs) if you reported any of these on your tax return:

• Nondeductible contributions made to a traditional IRA
• Distributions from a traditional, SEP or SIMPLE IRA that had nondeductible contributions (excluding rollovers, conversions, recharacterizations, qualified charitable distributions, one-time distribution to fund an HSA, or return of certain contributions)
• Conversions from a traditional, SEP, or SIMPLE IRA to a Roth IRA
• Distributions from a Roth IRA (other than rollovers, recharacterizations, or a return of certain contributions)

Click this link for steps on How to Trigger Form 8606.