If that doesn't work, delete the whole row by clicking the trash can at the right of the row and then click + Add Field and try to make that row again. It is ok for it to be at the bottom (the last row).
If that doesn't work, delete the whole row by clicking the trash can at the right of the row and then click + Add Field and try to make that row again. It is ok for it to be at the bottom (the last row).