Hello,
i use a Tacx Blue Matic and Wahoo RPM with Ant+
RPM show but Watt say 0
Whats wrong ?
Is that a Wahoo RPM cadence sensor or a Wahoo RPM Speed sensor?
You will need a Wahoo Speed sensor so Zwift can translate your read wheels speed into watts.
I have a Wahoo Blue SC Speed and Cadence Sensor
Ok, you originally said RPM.
Does the Wahoo Blue S/C register anything on the Wahoo Fitness App?
Could be a low battery or it is too far away from the wheel magnet.
The battery is new and the app is all set correctly.
He shows me only the cadence but not the watts.
I have checked the tire size, the USB extension cable and now I am at a loss
On the back of the Wahoo S/C make sure it says Ant+, some are just Bluetooth only.
We’ve outlined all the usual reasons won’t be able to move in this article: https://support.zwift.com/hc/en-us/articles/205703259-I-m-not-moving-in-game-speed-cadence-sensor-
Give those tips a try 
Ik heb een rpm speed& cadans sensor gekocht kan ik daar mee op zwift?