QB Issue Resolution:

When you are in the bill, down the bottom there is an attachment area, just click on the box and attach your supplier invoice and press save.

Resolution for Issue 'I want to attach a suppliers invoice to a bill' available: Yes (Solved).
Source: Intuit Community forum.

I want to attach a suppliers invoice to a bill: 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.

Support for attach a suppliers
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Error Recovery for attach a suppliers [Canada]
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