Does my grape juice have too much sugar to start fermentation?

1.100 is high but a good yeast should be able to tolerate it. If not, add more juice (being sure that there is no preservative in it) and see if it takes off. If it does, add more sugar. The best thing to do when the recipe calls for a high OG is to add half the sugar at the start and half after a week. BTW, you should not have to add yeast nutrient when fermenting grape juice. Save it for meads and country wines.