The Exercism page of How to learn Idris currently points to the official documentation of Idris1, not Idris2 which is located here. Since the Idris track in Exercism presumes the use of Idris2, you may want to fix the link at Exercism.
Cheers.
(As far as I can tell, Idris 1 is dead. The world has since moved on to Idris 2.)