Sounds like it is open already.

Use the open window list & close it. It should be under the view menu.

Resolution for 'book register issue: 'Why can’t I view the check book register in QB Pro 2017?' available: Yes.

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

For more information about resolving issues related to book register in Quickbooks/QBO or to get support for the issue 'Why can’t I view the check book register in QB Pro 2017?', click the link below: