It’s a new problem (as of last week), Intuit says they’re working on it.  In the meantime, you supposedly can create statements by click on the PLUS sign then OTHER.

Resolution for 'clients statements issue: 'I get an unexpected error while trying to generate clients statements.' available: Yes.

The clients statements 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 clients statements issue yourself by looking for a resolution described in the article. If it is a complex clients statements issue or you are unable to solve the issue, you may contact us by clicking here or by using other clients statements support options.

For more information about resolving issues related to clients statements in Quickbooks/QBO or to get support for the issue 'I get an unexpected error while trying to generate clients statements.', click the link below: