Converting from V-brakes to Disc Brake without existing mounting holes
In general, on electric bikes, the rear hub is an essential part of the electric motor/drivetrain in order to power the bike and is very different from rear hubs on standard 'manual' bikes.

Because of this, in order to have a rear disc brake system you'd need to replace your hub (or whole wheel if this isn't possible) with one that has disc brake rotor mounts. You won't be able to use the screw on aluminium one linked to in your question.

Secondly, you'll have to install a retrofit device onto your frame to mount the disc brake caliper. These are available with vary degrees of quality but looking at your frame, you'd need to remove the pannier rack which I assume you probably don't want to do.

I think it may be fruitless to pursue this conversion to be honest and if you have a disc brake on the front of your bike this is where to majority of effective braking should be happening anyway - simple rule is that you should use the front brake to slow your bike down until the point where you can use the back brake to bring your bike to a stop.