Without having received a distribution from the inherited Roth IRA and having no Form 1099-R to enter, I don't think that TurboTax can even know that you have an inherited Roth IRA, let alone calculate any RMD for this account. I think that you might be misinterpreting something in TurboTax. I've never encountered a situation where TurboTax offers to calculate any RMD for me.
