I'll take it as this is a new bank connection, useramanda-dolce.
I recommend validating the bank credentials first on your financial institution's website. Once verified, try connecting bank account again on your QuickBooks Online account by following the steps below:
From the Transactions tab, select Banking.
Click Connect account.
Enter or select your bank.
Enter the sign-in info you use for your bank's website.
When you're ready, select Sign-in.
Once connected, you can now categorise the transactions. I've added these links for more information: