Zwift has made some recent changes, not universally popular, to workouts. Have a read of this link for a lot more information:
Using the links I provided above you can, by searching for Red Unicorn, find that this weeks workout of the week is actually Red Unicorn Lite. It can be found under the Build Me Up Lite workout PLAN.
So you will probably not find it under the main list of workouts, in game, but instead need to check out the ‘Plans’ option, Enrol on the plan, select Red Unicorn Lite and do the workout. You probably then need to Cancel the Plan.
All a bit of a messy system.