Deadly,
There are many reasons that you might get this message.
I have never heard of an instance of RMXP / RGSSAD reporting this error.
It is typically a "Windows" error.
It could be a virus, anti-virus software, or file / executable permissions, or file accessability.
First, what version are you using? XP / VX? (Hit Help -> About, and copy the first line for an exact version)
What operating system? XP, Vista, ...?
Were you ever able to successfully playtest this game, or was this your first attempt?
You said your game was "finished", so I assume you've tested & save many times.
What did you do in the last editing session between the previous successful test, and the test that resulted in the error?
Did you add/modify any scripts? If so, which ones, and what did you do?
Can you list the custom scripts & any script edits that you have implemented?
Help us.... Help you! :scruff:
Be Well