Trial &1 is not assigned a planning resource

What causes this issue?

You have not specified a planning resource for trial &V1&. This value is
mandatory for trial planning.

System Response

The system does not create a planned order, since the trial is not
considered in trial planning unless it has a planning resource.

How to fix this error?

If you use trial planning, assign a planning resource to the trial.
If you do not use trial planning, change the configuration of the
<LS>Planning Resource</> field in Customizing.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.