Verify you have entered a form W2 in the Federal "Wages & Income" that has MO in box 15 with MO income in the corresponding box 16. Now in the MO state interview in the "Income" section you will be asked about the amount of those MO box 16 wages that are for remote work outside of MO. The amount you enter should be less than that MO box 16 amount (you mention 80%). That should work.
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