This is a merge issue caused by FamilySearch. They will fix it then a new update breaks it, we report it broken and then it keeps coming back. If you select any option with a radio button (circle) for the person on the right (middle column on that screen) it will throw that 400 error. You can select any checkbox option but not the radio button options.
Since you want to keep some of those radio button options I recommend for now that you add that fact to your RootsMagic database and then move it over to FamilySearch after the merge. To add the fact to RootsMagic click on the "View RootsMagic Person' button on the bottom of the merge screen. Then manually add that fact.