It's because you don't have the right system number. You must me on 2.3.320 or 2.3.340 for this to work. You're on 2.3.15. I used to have the same issues. Update your stuff.
If you have a stock backup you can flash back to that and then allow the ota update. Then make a fresh stock backup, delete the old, then flash whatever new rom you want!
If you don't have a stock backup then you have to either sbf or you can try the Maderstcok340.zip