"Calum is as tired as an old dog."
Translation:Tha Calum cho sgìth ri seann chù.
Why would it be? Chaluim is either genitive (and you wouldn’t say Calum’s is as tired… in English either) or a part of vocative – a Chaluim – the form used when addressing person directly. This sentence is about Calum, Calum is the subject, and needs to be in nominative, Calum.
This is explained in the tips and notes to the lessons. You can access them in the web browser version of Duolingo (unfortunately not in the mobile app) or on the duome.eu website.