Prior answer is correct.  Principal (corpus) payments from a trust are not taxable income and are not reported on a K-1.  The 1041 "may" have additional reporting when your Schedule B distribution Line 10 is greater than the Line 9 required amount.  Directly on the screen and the form it says on Line 11 "If greater than line 8, see instructions".

Also, you need to make sure the trust is allowed to distribute corpus.  Some trust documents only allow for income distributions.