No link between resources in operation &1 and &2

What causes this issue?

You want to release or schedule operation &V1&.

System Response

The resource in operation &V1& is not linked to the resource of the
preceeding or subsequent operation &V2& in the resoruce network of the
header.

How to fix this error?

Change the resource used for operation &V1&.

Error message extract from SAP system. Copyright SAP SE.