List.Average and Select rows

let
role = [ROLES]
in List.Average(Table.SelectRows(#"Changed Type",each [ROLES] = role)[WAGE])

I simply updated this part of your formula with the correct syntax:

each [roles] = _[roles]

[ROLES] is a column assigned to the role variable. This should be done whenver you're trying to access a column outside the current scope which in this case is #"Changed Type". Whatever the value of ROLES is it will be assigned to this variable and will be used to filter the previous step which result is carried over in the current step. By not doing this you are simply telling Power Query to return true for all rows.