Good Morning,
I have started yesterday to train with zwift and I’m too slow. With my correct waight I have only an speed of 8-12 km/h .
My Setting:
Elite Smart B+ Trainer
Cross bike with I think 30x622 Wheel
Huawei Mate 20 Pro shared on Beamer for Zwift
When I’m driving normal (outdoor I have a average between 21-24 km/h with this power). Zwift tells me, that I have between 80-100W and only 8-12 km/h
I have to set my wight between 50-60 kg to drive normaly my 21 km/h.
What do I wrong?
First read this: https://forums.zwift.com/t/how-does-zwift-determine-my-speed/
Second, make sure that you have properly calibrated your trainer using the Elite app.
Can you tell me, how I have to calibrate it? I have no manual to it 
What are you inflating the tires to?
What speed and cadence sensor are you using?
Also make sure you are selecting the correct trainer on the Pairing Screen.
Have you seen this video with instructions about how to calibrate your trainer within Zwift? I can’t insert the link, but it’s called: How to Calibrate Your Smart Trainer — Official Instructions | Zwift
Also, as others mentioned, find out which tire pressure is recommended for your trainer and make sure you’ve got that right.