I did not take the RMD for 2020. It was suspended for 2020. What do I put in the box for RMD amount?

The RMDs have been waived for tax year 2020.

Please read this IRS document for more information.

If you did not take a RMD in 2020, then there is nothing to report about your RMD on your tax return. There is no penalty.

...And if you have an unused 1099-R form for that issuer, you delete the 1099-R involved in the retirement income section.