can i use zwift with just a speed sensors and does it still work properly
Yes. You use what’s referred to as Zpower where you select your trainer on the pairing screen along with your speed sensor. So long as your make and model of trainer is listed you can Zwift with it. Zpower is used on “dumb” trainers using a known power resistance curve. This estimates your power. Not accurate but provided you pump your tyre up to the same pressure each ride the power numbers should be consistent. Great for workout training and just free riding.
If you want to race on Zwift then you should invest in a smart trainer and or a power meter.