If your frame shift drive has a lower range than your target destination you will get the warning you are seeing, it can also happen when you are fully laden, due to the additional weight, your FSD capabilities are lower.
Instead of trying to jump directly to your destination, plot a course via the galaxy map.
Open the galaxy map, choose where you want to go by clicking the star...a small box will pop up to the right.
Click the Plot button and it should draw an orange line showing the route it will take you on.
If the option is red, make sure its < 100Ly distance.. the planner only works to that range so far.
Exit the galaxy map and it should set your first jump.. once you make the jump it should target the next.. and so on.
If you target anything else during this, you can get back to the planned route via the left navigation pane, in the list will be the next destination with an icon resembling a linked route (looks like the steam icon a bit)
I think that covers it...Hope that helps.