HELP !! Sum formula with different variables

the join between tables is active, 1:many and with single direction?
you added the new column to the Adressen table, correct?

EDIT - maybe there are some spaces/non printable characters in the Gebiedstype column?  try this code

Average =
IF (
    Adressen[civiel gesloten] <> BLANK ();
    SWITCH (
        TRIM ( Adressen[Gebiedstype] );
        "Kern"; RELATED ( Lengtes[Gemiddelde graaflengte Kern] );
        "Buitengebied"; RELATED ( Lengtes[Gemiddelde graaflengte Buitengebied] );
        "Industrieel"; RELATED ( Lengtes[Gemiddelde graaflengte Industrieel] );
        BLANK ()
    )
)