When you are connecting the edges you have to use proper lattice index. Here is a working example for armchair (V1.0.1).

from math import sqrt

import kwant

from matplotlib import pyplot

# Define the graphene lattice

sin_30, cos_30 = (1 / 2., sqrt(3) / 2.)

#armchair

graphene = kwant.lattice.general([(0, 1), (cos_30, sin_30)],

                                 [(0, 0), (1 / sqrt(3), 0)])

a, b = graphene.sublattices

hoppings = (((0, 0), a, b), ((0, 1), a, b), ((-1, 1), a, b))

def aclead(w, onsite=0,t=-1):

    sym = kwant.TranslationalSymmetry(graphene.vec((-1,2)))

    def lead0_shape(pos):

        x, y = pos

        return 0<=y<w+0.6

    lead = kwant.Builder(sym)

    lead[graphene.shape(lead0_shape, (0, 0))] = onsite

    lead[[kwant.builder.HoppingKind(*hopping) for hopping in hoppings]] = t

    lead[b(0, 0), a(w, 1)] = t

    return lead

def acsys(w, onsite=0,t=-1):

    def box(pos):

        x , y = pos

        return -0.1<=x<1.2 and 0<=y<w+0.6

    sys=kwant.Builder()

    sys[graphene.shape(box, (0, 0))] = onsite

    sys[[kwant.builder.HoppingKind(*hopping) for hopping in hoppings]] = t

    sys[b(0, 0), a(w, 1)] = t

    return sys

for w in range(4,7):

    sys = acsys(w)

    sys.attach_lead(aclead(w))

    sys.attach_lead(aclead(w).reversed())

    kwant.plot(sys)

    lead = aclead(w).finalized()

    kwant.plotter.bands(lead)