The response when I submitted a bug report for the insertion problem is that Legacy does not support FGRs for unmarried individuals. This was determined notwithstanding the fact those reports can be generated either as part of or not part of a book in the Publishing Center and they function perfectly well. If you wish to insert a FGR for an unmaried individual, it may work. If not, the following workarounds seem to work when a simple insert will not.
1. Position the "Family" tab to the unmarried individual you want to include.
2. Click the "Reports" icon.
3. Click the "Publishing Center" button.
4. Notice the "Current Person" is the individual chosen in the Family tab and the "Current Couple" says "None".
5. In the "Chapters of Book" window on the right, navigate to and click the chapter BEFORE which you want the new chapter inserted.
6. In the "Available Items" window, click the Individual Report selection.
7. Click the "Insert" button.
8. Leave the new chapter selected, and click Family Group Report in the "Available Selections" window.
9. Click the "Insert" button.
10. Select the unwanted Individual Report chapter in the "Chapters of Book" window
11. Click the "Remove" button.
You can also perform the following workaround which has never failed me.
Follow steps 1-4 and then
5. Click Family Group Report in the "Available Items" window.
6. Click the "Add" button.
7. Click the new chapter added at the bottom of the list in the "Chapters of Book" window.
8. Use the up and down arrows to position the new chapter to the proper place in the book.