The code C for box 18 designates this $31 as "nondeductible expenses paid or incurred by the partnership....not deductible on your tax return. Decrease the adjusted basis of your interest in the partnership by this amount". So, not sure why it is causing a "tax-exempt" error, except that box 18 codes A and B related to tax exempt income.
I recommend two things. First, try entering the code C and amount on the third line of that schedule on the box 18 page. If that doesn't work, try leaving the $31 box 18 code C amount off the K-1 entry. It is not deductible on your return, and as long as you take it into account for your adjusted basis calculations, you don't need it for your tax return calculations.
See the IRS Instructions for box 18 of Form 1065 Schedule K-1 at this link.
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