How to I stop getting an error to connect an existing previously-working bank account that is no longer syncing?

Hi there, Ed.

The error code (590) you've encountered while reconnecting your credit card account in QuickBooks Self-Employed indicates a temporary disruption in the connection between your bank and the program. No worries,  this will self-resolves and doesn't last more than 48 hours.

I recommend waiting a few hours to see if it resolves itself. Then, connect your credit card again. Here's how:

1. Go to the Gear icon and select Bank accounts.
2. Choose Connect another account and enter your bank's name or URL in the Search bar.
3. Follow the on-screen process of connecting an account.

Additionally, I'm adding this article to guide you in reclassifying your transactions accordingly in the program: Categorize transactions in QuickBooks Self-Employed. 

I, along with the Community team, are always here for you. If you have more questions about your bank or credit card connectivity in QuickBooks, kindly reply to this thread. We'll be around the clock to promptly lend a hand.