What causes this issue?

A material component is not included in an availability check, if
its committed quantity was changed manually in
<GL:backorder_processing>backorder processing</> and
the availability check refers to <ZH>all</> material components in the
order.

How to fix this error?

To check the availability of a component whose committed quantity was
changed manually, proceed as follows:
Branch to the component overview screen.
Select the desired component.
Check the availability of the <ZH>single</> component by selecting the
menu options <ZH>Component -> Check availability</>.

Error message extract from SAP system. Copyright SAP SE.