What causes this issue?

In <ZH>production control profile</> in the current order it is
specified that in the material availability check the confirmed
quantity depends on the overall availability of the order (indicator:
confirm available partial quantity).
In an <ZH>individual check</> the confirmed quantity cannot be
calculated since not all the components are checked.

System Response

The maximum quantity to be confirmed is displayed for information
purposes.

Error message extract from SAP system. Copyright SAP SE.