Planned order &1 created as an SNP planned order

What causes this issue?

In SAP R/3, the indicator <LS>Create Planned Orders as SNP Planned
Orders</> was set when an integration model was activated for planned
orders (CFM2). In SAP APO, a planned order transferred from SAP R/3 is
created as an SNP planned order if the following prerequisites are
fulfilled:
The planned order does not yet exist in SAP APO
The MRP procedure X (without MRP, with BOM explosion) is assigned to
the header material of the planned order in SAP R/3
The availability date/time of the header material is outside the
<DS:GLOS.363D6D1E6AE92097E10000009B38F984>SNP production horizon</>.
It is neither a planned order of the type VP (forecast), nor a planned
order for project stock or sales order stock.
In the product master (transaction /SAPAPO/MAT1), the SNP production
horizon or the SNP production horizon extension has been maintained on
the SNP 2 tab page.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.