Two things, first of all in order to day trade you need to change your account to a margin account. Just because you have a margin account though doesn't mean you have to "trade on margin". Trading on margin occurs when you use your margin account to borrow money in order to buy stock. To avoid trading on margin, just make sure you don't invest any more money than you have deposited in the account (it's that simple). Without a margin account however, all trades must settle before you can place another trade. It takes 3 trading days for a typical stock trade to settle, so you won't be able to do much daytrading.
Secondly there is a SEC (Securities Exchange Commission) regulation that says you cannot daytrade unless you have at least $25,000 invested in your account. USAA must comply with this regulation, so make sure you have $25k in the account otherwise USAA won't let you daytrade.