How to define a lower triangular decision matrix

[odow]

You were pretty close:

using JuMP
import LinearAlgebra
model = Model()
@variable(model, A[1:3,1:3], Symmetric)
Al = LinearAlgebra.LowerTriangular(A)

julia> @expression(model, Al * [1, 2, 3])
3-element Vector{AffExpr}:
 A[1,1]
 A[1,2] + 2 A[2,2]
 A[1,3] + 2 A[2,3] + 3 A[3,3]