@clubstew Have you tried restarting it? I can get it to interact after doing so; unfortunately, my bug doesn't allow me to finish any cairns in the game (on PC).
Same thing happens to me since I did the Yule drinking event last night.
I have tried all the tricks to get cairns to work - restarting the game from desktop, restarting from title, holding 4th rock 30 seconds before placing - and I can't get it to validate and let me progress in this questline. Any suggestions?