I solved it, because of the naming of my columns I needed to put the formula as:
Mean(
:Name( "1L" ),
:Name( "2L" ),
:Name( "3L" ),
:Name( "4L" ),
:Name( "5L" ),
:Name( "6L" ),
:Name( "7L" ),
:Name( "8L" ),
:Name( "9L" ),
:Name( "10L" ),
:Name( "11L" ),
:Name( "12L" ),
:Name( "13L" ),
:Name( "14L" ),
:Name( "15L" ),
:Name( "16L" ),
:Name( "17L" ),
:Name( "18L" ),
:Name( "19L" ),
:Name( "20L" ),
:Name( "21L" ),
:Name( "22L" ),
:Name( "23L" )
)