You can reset sales by going in to Administrator > Reset and select which data you want to reset.
Some time, after you hit the Reset Sales, this error pops up:
When this happen, return to Cash Register screen, and check if you have any transactions left. Make sure you check both of them, the Tables and the Take Away. Call out any transactions that remains there, and Checkout or Void them.
If you still can't reset, write to us at support@arms-fnb.com.
ps: this function will be improved in the next version.