My web search for LC-50LB8601U only finds this post. Did you enter the model correctly? The initial announcement said Airplay support was being added to Rokus supporting 4K - I've not seen anything saying they've extended to lower resolutions, although I could have missed it.
Roku lists the devices below as supporting streaming with AirPlay 2:
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking " Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X