If, Or, Else not working

use PowerQueryFormatter.com to bring your code into a readable format

let
  a = 
    if List.Contains({"Under 18", "18-21", "22-29", "30-34", "35-39"}, [Q1_Age])
      and (
        {
          [#"Q6_Employment_Working full time (30 hours or more work per week)"]
            = 1 or [#"Q6_Employment_Working part time (under 30 hours work per week)"]
            = 1
        }
      )
      and ([Q2_Parent_None of these apply] = 1)
      and List.Contains(
        {"Single/Never married", "De facto/Living with partner but not married"}, 
        [Q3_Marital_Status]
      )
    then
      "SEGMENT A"
    else
      null
in
  a

Then validate if that is the actual logic you wanted to implement