It's hard to tell exactly why this might occur unless you know what's included in the rest of the worksheet. Are there expenses in section E? The way shared residency is calculated, only one parent pays, so I'm a little confused. Can you share some more details?
What software has been used to create the worksheet? In general, a negative amount on line F6 would simply be ignored, but it should be equal and opposite for each parent. For instance if parent A shows positive $213 (paying), then parent B would be ($213) (receiving).
