The intended way of doing that with inla.spde.make.A() is to define a
mapping between continuous space/time and the discrete model
representation using a "1d mesh".
The default is to assume that the user has already computed that
mapping, and that is why you get the error.
In your case, and using the latest inla version, you can do this
directly in the inla.spde.make.A() call:
inla.spde.make.A(...,
group=time,
group.mesh=inla.mesh.1d(time, degree=0))
In older versions, you need
inla.spde.make.A(...,
group=time,
group.mesh=inla.mesh.1d(time),
group.method="nearest")
Finn