Material group & cannot be converted

What causes this issue?

The conversion rule for material group &V1& is unknown.

System Response

The relevant record cannot be prepared in POS - interface outbound
processing. The relevant document cannot be posted in the SAP system in
POS - interface inbound processing.

How to fix this error?

<DS:SIMG.W_DF_VK_0123.SIMG_OPOS>Define a conversion rule for the
material group </>.

Error message extract from SAP system. Copyright SAP SE.