Why is "don't" translated as "na" here instead of "cha(n)"?
Because it's a command. Cha is used with active verbs.
If, for example, this phrase were "Alastair was not worried" you would use cha then. That would be "cha robh Alastair a' gabhail dragh".
Na gabh dragh is what I entered. Duo said that it was wrong and the answer was Na gabh dragh. No spelling errors or any variation from my input?
Happens sometimes (rarely). Also bugs content appear in the app that don't appear in web browser and vice versa.
Did you put alasdair at the end?
What's wrong with "Na gabh dragh ort, Alasdair" here?
'Ort' as in 'Alasdair is wearing'?
As in "upon you" - "ort" is derived from "air a tu" meaning "on you". The "wearing" meaning is secondary.
Maybe it's wrong because we've only covered the 'wearing' meaning?
so, don't worry is literally "don't take apart"?
kinda like, 'dont think about it', is that the idea?
Wiktionary says gabh is take, receive, and dragh is 1. annoyance, trouble or 2. worry, anxiety.