You want
julia> testList = Vector{Matrix{Float64}}(undef, 0)
Matrix{Float64}[]
or
julia> Matrix{Float64}[]
Matrix{Float64}[]
This is just a type, not an instance of the type
You want
julia> testList = Vector{Matrix{Float64}}(undef, 0)
Matrix{Float64}[]
or
julia> Matrix{Float64}[]
Matrix{Float64}[]
This is just a type, not an instance of the type