If the interest isn't exempt in Minnesota, you need to indicate that on the federal schedule B.  From your post, it's not clear if you are doing that.

It is on Federal Schedule B and flowing to the Minnesota return Schedule M1M line 2 which then flows to M1 line 2.

I'm not really following your original post about line numbers, but are you itemizing on either the federal or state returns?

No. The taxpayer was a part year resident of Minnesota then a resident of Kansas. The tax exempt income was Kansas based causing it to be an addition to Federal Taxable Income totaling on line 3 of the Minnesota M1.

The error detail states:

On form M1, line 8 Minnesota Taxable Income must equal line 4 Adjusted Federal Income minus line 7 subtractions to line 8 must not be negative.

Problem is that line 9 is Minnesota Taxable Income which includes line 1, Federal Adjusted Gross Income + non-Minnesota tax exempt interest from Federal Schedule B. That total is on line 3 of the M1. Proseries is taking M1 line 1 - M1 line 4 = Minnesota Taxable Income. It should be M1 line 3 - M1 line 4 = Minnesota Taxable Income.

My MN returns all flow properly so I don't know what is going on with your return.  You might have to call support.  But out of curiosity, how much interest are you talking about for the Minnesota add back?

A whole $209 😞

For that amount, I wouldn't treat it as an addback.  For the couple bucks in tax, they can lock me up and throw away the key if they want, but it isn't worth the fight with the software.