Validation rules are not intended for the purpose you have posted. These are intended to ensure that invalid data entries are not made (like a negative number when only positive numbers should be used or that date entries make sense.) What you have here would prevent data entry when the value in the current record field [Quantity]<10 which doesn't make any sense from reading your other posts (assuming that this is for your inventory project :) )
You really need to read thru the FAQ links in my first reply to your thread.
Once again, this will be a either an Access Macro or a VBA coding. You can do this within the form or (if you ACC2010 or newer) via table level macros as a sort of trigger.
IN any-case, the code will need to take your current entry, compare it to the formula based on your data, and then provide a prompt if needed.
We need some info about versions of the OS, Office/Access, tables and their relationships. Otherwise this thread will dead-end quite quickly.
WE do want to help; however, we need you to help us to help you by providing enough detail.