Spin bikes are not meant to work with Zwift, people have hacked together ways to get them to work but it really isn’t the best experience.
If it’s not too late, return the spin bike and the speed sensor and check out the current deals on smart trainers. Note the prices on Wahoo items listed in the article below are incorrect, but I think the Wahoo Snap is $350
I assume you have a speed sensor that use a accelerometer. Those sensors are made to be attached to the hub of a wheel, it is not designed to be mounted on it’s side.
But please take note of the post above by @Mike_Rowe1 .
In addition to what Gerrie said, it makes sense that the cadence sensor works as those are intended to be mounted on the side of a crank arm, so the internal accelerometer expects to be moving sideways.
I have a spin bike with a wahoo speed sensor on the flywheel which works, its important to not place the sensor on squarely, make sure its rotated 15-20 degrees clockwise and it might work, you might need to experiment with the amount of misalignment, tricky I know when using super sticky tape to attach the sensor