It’s a script error.  You must report it to QuickBooks.

See below.

https://community.intuit.com/questions/1640087-banking-error-9999-from-wells-fargo-this-has-been-goi…

Resolution for 'accounts with QB online issue: 'I have been getting below error on both of my accounts with QB online for the last 4 days. “Sorry, we can’t update your account. Please try updating again later. (9999)”' available: Yes.

The accounts with QB online issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the accounts with QB online issue yourself by looking for a resolution described in the article. If it is a complex accounts with QB online issue or you are unable to solve the issue, you may contact us by clicking here or by using other accounts with QB online support options.

For more information about resolving issues related to accounts with QB online in Quickbooks/QBO or to get support for the issue 'I have been getting below error on both of my accounts with QB online for the last 4 days. “Sorry, we can’t update your account. Please try updating again later. (9999)”', click the link below: