What causes this issue?

The missing parts list shows the result of the last overall
availability check.
No missing parts list can be displayed for your current order (&) for
one of the following reasons:
Within the transaction no overall availability check has been found
yet.
The order was not a part of the last overall availability check
(for example, in the <GL:collective_order>collective order</>).
After the overall availability check an individual check was carried
out. This individual check deletes the result of the last overall
availability check.

How to fix this error?

Carry out an availability check for the order.

Error message extract from SAP system. Copyright SAP SE.