Handling unit &1 has status &2 - No Assignment to outbound delivery

What causes this issue?

While processing an <DS:GLOS.352A9250C1035CD5E10000009B38F974>outbound
delivery</>, you can only assign handling untis that are currently in
stock. This includes handling units with status '0020' = (stock not at
HU STORAGE LOCATION) and '0030' (stock at HU STORAGE LOCATION).
Handling unit &V1& that you selected has a current status of &V2&,

System Response

Handling unit &V1& cannot be assigned to this outbound delivery.

How to fix this error?

If the handling unit has status '0001' (planned) or '0010'-'0011'
(realized), check whether you have to post goods receipt for the
handling unit. If the stock is already in the warehouse, you can change
the status manually by choosing 'Process handling unit'.

Error message extract from SAP system. Copyright SAP SE.