990T taxes on 401K and IRA when not withdrawn

i have 3 partnerships in my 401K, and i was told that any money made in these funds would not be taxed until i take the money out.
Schwab sends me a 990 T called exempt organization business income tax return, and I assume it may be related to a partnership ETP, MAin or ET. I have REIT ETF with Vanguard, but they do not send me this type of paperwork.
I am selling these to eliminate the paperwork in the mail.
Why am i under the impression that what is in 401K or I RA, is not taxed until you take it out, therefore eliminating all the tax hassle.