Both of the following analyses result in the message in the printscreen. What am I doing wrong here?

Syntax:

*deelvraag 2 MODERATIE :versterkt of verzwakt sociale steun (moderator) verband tussen interactie met huisdier en expliciete eenzaamheid.
MIXED mood_lonely with pet_interact age gender education SessionInstance
    /CRITERIA=CIN(95)
    /PRINT=SOLUTION TESTCOV
    /METHOD=REML
    /FIXED=INTERCEPT pet_interact pet_interact*MSPSS_all2 SessionInstance
    /RANDOM=INTERCEPT pet_interact | SUBJECT (subjID)
    /Repeated=SessionInstance | SUBJECT (subjID) COVTYPE (AR1).

*deelvraag 2:versterkt of verzwakt sociale steun (moderator) verband tussen interactie met huisdier en impliciete eenzaamheid.
MIXED Impliciete_eenzaamheid with pet_interact age gender education SessionInstance
    /CRITERIA=CIN(95)
    /PRINT=SOLUTION TESTCOV
    /METHOD=REML
    /FIXED=INTERCEPT pet_interact pet_interact* MSPSS_all2 SessionInstance
    /RANDOM=INTERCEPT pet_interact | SUBJECT (subjID)
    /Repeated=SessionInstance | SUBJECT (subjID) COVTYPE (AR1).

Hi. The first thing I notice is that the variable MSPSS_all2  does not appear in the MIXED statements:

MIXED mood_lonely with pet_interact age gender education SessionInstance

-- or --

MIXED Impliciete_eenzaamheid with pet_interact age gender education SessionInstance

Thanks, it is working!