Display Error: Error: method not implemented

New to Julia
1

Hi

I am following a turotrial from here ⚡ Pluto.jl ⚡

when I construct a struct and try to instantiate it I get the following error. Display Error: ERROR: method not implemented Any idea what the issue is?

using ReingorcementLearning: AbstractEnv

mutable struct BlackjackEnv <: AbstractEnv
    dealer_hand::Vector{Int}
    player_hand::Vector{Int}
    done::Bool
    reward::Int 
    init::Union{Tuple{Vector{Int}, Vector{Int}}, Nothing}
end

BlackjackEnv([],[],false, 0, nothing)

>>> Display Error: ERROR: method not implemented