I believe it's 10.11, per @kevinushey's answer here, too.
I don't have 10.11 to test, but hopefully the IDE team can clear this up. There was an unintentional 10.12 requirement, but I believe that's been patched:
I wonder if it's fixed in the dailies but not in preview for some reason:
https://dailies.rstudio.com/
Thanks for reporting.