QB Issue Resolution:

in company settings, insure you have track qty on hand set to on

and in company settings?form delivery?customize check to insure those fields are checkmarked for display and print

on the product itself insure the box is checked for I track qty or something like that

are all those settings correct?

Resolution for Issue 'why am I no longer able to assign a quantity to a product or service?' available: Yes (Solved).
Source: Intuit Community forum.

why am I no longer able to assign a quantity to a product or service?: 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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