I have just finished a live chat with Samsung Support and after some testing, I was told:
"Thank you so much for patiently waiting. Upon checking, we do apologise but at Sidesync is not compatible with S9 at the moment. Samsung are working on it as soon as possible for more improvements."
Before contacting samsung, I tried:
-Re-installing SideSync on PC
-Re-Installing Samsung Mobile USB drivers
Samsung adviser asked me to try:
From the dialler on your device, enter *#0808#.
Under 'USB', select AP.
Under 'USB Settings', select MTP +ADB.
Soft reboot device