Transfer of nondeductible IRA funds to a solo 401k

[Jim P.]

I have a Traditional IRA that includes some contributions during years when the contribution wasn't tax-deductible because I or my wife had 401k accounts.

Last year I did a rollover of the entire Traditional IRA into my Solo 401k, and then did an in-plan rollover of my entire Solo 401k into a separate Roth-designated Solo 401k account.

I'm trying to figure out how to get TurboTax to understand that part of that in-plan Roth rollover shouldn't be taxed because some of the original Traditional IRA money was already taxed (and I haven't yet submitted the 1099-R for that rollover, so I need to do that).

Then I came across this...

https://www.mysolo401k.net/nondeductible-ira-contr...

That article claims that you can't actually rollover nondeductible traditional IRA funds into a Solo 401k at all. Is that true?? I can't find any other source confirming that. If that is true, what am I supposed to do now?