Material group & not defined (please check your input)

What causes this issue?

The <GL:material_group>material group</> you entered has not been
defined.

How to fix this error?

INCLUDE EINGABE_PRUEFEN_NA LANGUAGE EN
DEFINE &OBJEKTE& = 'material groups'
INCLUDE NA_EINK_CUST LANGUAGE EN
IF &DEVICE& = 'SCREEN'
<DS:SIMG.SIMG_CFMENUOLMSOMSF>Define material groups</>
ENDIF

Error message extract from SAP system. Copyright SAP SE.