I got the same error. I found a government website saying you get this error if "filing a pre-bankruptcy return but there is no date of bankruptcy" on record. But that makes no sense here so it must be something else. Had a somewhat complex return with capital gains, dividends, pension income etc but it worked fine last year. Medical expenses were big, cuz physically disabled and living in a nursing home, but, again although it was more expensive in 2020 cuz was in the home all year, not clear why cannot file electronically this year.
