QB Issue Resolution:

there is no sales orders in QBO, are you using an estimate for that?

allocate means different things to different people, if you mean reserve a qty of an item for a customer, you can not

inventory is a bucket, and inventory is only used when you put it on a customer invoice and save the invoice

Resolution for Issue 'How can I allocate incoming inventory to sales orders?' available: Yes (Solved).
Source: Intuit Community forum.

How can I allocate incoming inventory to sales orders?: 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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