As per title, been unable to uturn when i am on climb portal rides. Unable to do so on both my primary device (samsung s8 plus tablet) or my companion app (flip4).
The only way i regain my uturn button functionality is if i teleport to a pacer group.
Welcome to the forum.
You need to ride above ±2km/h (can’t think of the exact number) to be able to u-turn.
https://support.zwift.com/en_us/performing-u-turns-in-zwift-HyEMnL_Vr
The official line is “3mph (~5 kph)” or higher. 
I’ve noticed a few things.
1 you must hit and hold the uturn key. It no longer works if you merely tap it.
2 I find I have to do 100 watts to turn.
3 you will lose your time and progress. All of the above was done to prevent accidental upturns.
thanks all, weird though as I was definitely doing more than 5kph when “mashing” or holding on to the uturn button.
Will try again soon in a ride.
And when I did a u-turn on Puy de Dome, it was like doing a workout on AdZ that bleeds into the downhill. I had to really put out the watts, there was no coasting at all. Maybe it’s been fixed or maybe it’s supposed to be that way, but I was surprised it was so hard to go down such steep hills. 