I can't buy from the Online Store.

It won't let me buy the bonus mission pack..
It says something about security.
Nothing is wrong.
Is it because I'm using a different email to the account?
It's the only thing I can come up with.
Although it's my brother's account and I kinda stole it.
Because we've used 2 different PCs, may they be not letting me buy?

it could be the card you're using...I know it gives that error for certain types of visa gift cards...

I think it prevents you from making 2 purchases within 24 hours for credit-card fraud reasons. Could be wrong though.

its a security measure to prevent people from using your card to buy 2 items within 24 hours, wait a day and you should be able to buy a new one

Okay thanks guys, I'll try later when my mom get's home.

This looks like a poem

I had the same issues, but I didn't buy anything for months when I wanted my Character slot (rule of 2 buys in 24hrs).
My card was getting rejected all over again - it was a Mastercard. The issue didn't resolve, but I will "have to" try again sometime soon.