Yes, "Vart ska du?" is correct since "vart" indicates a direction and "ska" can be used to mean "go".
"Var ska du?" is an incomplete sentence since "var" indicates position. You would have to add another word to make the sentence complete.
For example, you could say "Var ska du sova" - Where will you sleep?
In words like 'bort', adding an 'a' or other similar changes at the end signifies being at rest, while keeping the word unchanged means there is movement.
For example, 'Jag ska gå hem' means 'I will go home' (movement), while 'Jag är hemma' means 'I am at home ' (stationary)
'Hon gick långt bort' means 'She went far away' (movement), and 'Hon är långt borta' means 'She is far away' (stationary)
Hope this helped!
Whenever you get an answer wrong (or, rather, the system thinks you do), a button pops up that lets you submit error reports. On desktop, it says "report a problem". On mobile, it's a flag icon beneath the comments icon. Since contributors cannot always monitor threads as well as they'd like, the preferred ways of reporting errors is using that button. This way, it gets submitted to the right place and they can look at it when able to.