Problem with new bicycle front led light and old bulb back light
You haven't linked to the datasheet for the front lamp so we can't be precise here but there are a couple of potential problems.

Bike 'dynamos' are actually an alternators. They output alternating current. They also carefully design the coils so their inductance causes an increasing voltage drop (in the alternator) with increasing frequency. The result is, that when the intended incandescent lamps are installed, that the brightness is pretty good at quite low speed but you don't burn the lamps out at high speed. That is, the alternator output voltage is reasonably constant over a wide range of speeds when the correct load is applied.

Parallel connection of the lamps is correct. Both should be paralleled off the alternator. My LED tail-lamp is connected to terminals on the headlamp. I wouldn't connect an incandescent bulb to those terminals. Try connecting directly to the alternator.

If that doesn't work your LED headlamp may be pulling down the voltage enough that the rear lamp isn't working. Best bet would be to buy the matching LED tail lamp.