if you try to attack someone without oil and then you buy some you still can't attack them for whatever reason the game thinks you still don't have any oil but it seems to only bug out against the person you originally targeted.

is that by design?

I think I have had this happen on the 1st reattempt after purchase of oil, on the 2nd it worked.

game allows to attack because the attack window is not updated with your recent purcahse.

I have had that happen a few times.

Refresh the window, works for me!

You can click the refresh army link.