I want to automatically sync my transactions with my quickbooks cash account to itemize them but it didn't let me the first time because of some error. How do I?

Did you maybe make a mistake in entering the routing and account numbers for your "cash" account?

But if you mean setting up your sales receipts and received payments to autopost to an internal cash account (cash drawer, petty cash, etc) you can specify the default account that payments post into instead of using Undeposited Funds.  In fact it will post to the last account used unless you change it