You can not.
QB is accounting and you are asking about company policy decisions. About all you can do is print the PO, send it to each person needing to sign off on it, and when you get it back complete the order.
Resolution for 'back complete issue: 'How to approve purchase invoices in the system by people outside an organisation' available: Yes.
The back complete 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 back complete issue yourself by looking for a resolution described in the article. If it is a complex back complete issue or you are unable to solve the issue, you may contact us by clicking here or by using other back complete support options.
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