QB Issue Resolution:

We can remove the box by closing all windows. To do so, click the Window menu, select¬†Close All. Let’s create the Purchase Order afterwards.

 

You can check this reference about the Accounts Payable workflows and in managing your preferences.

Resolution for Issue 'Add item while in Purchase Order' available: Yes (Solved).
Source: Intuit Community forum.

Add item while in Purchase Order: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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