Other exercises sugar is spelled like: sìugar
Is this an exception or is one of them a mistake?
Nope, there aren't any sentences on the course with "sìugar" :)
Is "Tha seo sìucar agus bainne" not a valid way to say it?
"Seo" (when introducing something) means "this is", so the "tha" would be redundant.