If you're unable to download the transactions, let's make sure to sign in to your bank or credit card's website to check if there are any system maintenance.
If there's none, we can do the manual update by following these steps:
Otherwise, read this article to fix any errors you encountered: What to do if you get a bank error or can't download transactions in QuickBooks Online.
Please feel free to add the name of your bank and other details in the comments below. This way, I check our records for any investigations. Thank you in advance.