I've been scratching my head on this one for a few weeks. I worked around this problem for my situation by setting the cost basis to $0 on the IRA Information Worksheet, since I'm currently only taking withdrawals from my inherited IRA, which has no cost basis. The cost basis of my other traditional IRAs won't be needed for many years, until I start taking RMDs from these accounts.
Oddly, I didn't have this problem with TurboTax from 2012 through 2019, even though it was keeping track of the cost basis of my other IRAs, but this did not affect the inherited IRA calculations. Apparently this bug was introduced in the 2020 version.
Hopefully by the time I need to take RMDs from my other IRAs, TurboTax will separately handle any inherited IRA cost basis as well as traditional IRA cost basis, as specified by the IRS. From what I've read, inherited IRA cost basis must be maintained separately unless the IRA is inherited from a spouse.
