What causes this issue?

The operation does not have any capacity requirements due to one of the
following reasons:
There are no capacities defined at the work center at which the
operation is carried.
The control key of the operation specifies that no capacity
requirementsare created.
The operation has not been scheduled.

How to fix this error?

Check the operation and the corresponding work center:
If necessary, change the settings so that capacity requirements are
created for the operation.
If necessary, scheduled the order.

Error message extract from SAP system. Copyright SAP SE.