Problem with MO-NRP (Missouri)

I had filed my Federal Business Tax and I was trying to fie the Missouri tax form. It said I had to mail it in but then it said it had problems. It highlighted a line called Missouri Source % (below Line 13) , and it said it had to be filled. This line was showing it was line 13 divided by line 12. Unfortunately, line 12 was completely greyed out for me. Apart from not knowing what to put in that cell, the cell would not even let me type anything inside it. I had just wanted to type 100%. Does anybody know how I should resolve his. Any help would be appreciated. Thanks.

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