Not a valid number
Either you've just typed in, or else Oxford WordSmith Tools has just attempted to read (e.g. from wordsmith.ini, the defaults file), something which is expected to be a number but wasn't. Computers will not see the capital O as equivalent to the number 0. Or else there is a number but accompanied by some other letters or symbols, e.g. £30. If this happens when WordSmith is starting up, check out the wordsmith.ini file for mistakes.