You cannot choose joint production here

What causes this issue?

You wanted to display or maintain data on
<DS:GLOS.joint_production>joint production</>. However, this is not
possible since the material has neither been flagged as a
<DS:GLOS.co-product>co-product</> nor is it a process material.

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?

Either set the <LS>Co-product</> indicator on the appropriate
<LS>MRP</> screen or, if the material should be a process material,
create it with the <DS:GLOS.material_type>material type</> <LS>PROC</>
for process materials.

Error message extract from SAP system. Copyright SAP SE.