I tried looking up the 590-B worksheet mentioned since lines 6-12 on 8606 were blank for me as well.
It looks like 590-B still isn't finalized for 2020. Draft isn't available either.:
https://www.irs.gov/forms-pubs-search?search=590-B
https://www.irs.gov/pub/irs-pdf/p590b.pdf
https://www.irs.gov/pub/irs-dft/p590b--dft.pdf
Maybe that's why the error is occurring.
