"I do not know why you do not like him."
Translation:Ich weiß nicht, wieso du ihn nicht magst.
Yes, it should probably better be "du". One used to capitalize the informal you as well, as a from of respect, until there was a reform some years ago. I believe both forms are acceptable these days, so "Du" is not wrong.
Personally, I actually like the old-fashioned "Du" when writing a letter or personal message to someone, but it's not that common anymore.
Because in this case when you have a middle word 'handle' like why or while etc in the middle of the sentence I believe this indicates a new clause. This second clause is subordinate to the first clause "I don't know" - as 'why you don't like him' wouldn't make sense on its own without it.
When this occurs, the verb gets thrust to the end. This is why the 'magst' in du magst gets thrown to the end.
I believe this is right but I'm not 100% sure, seek clarification!
Ah thanks, I didn't think about that. I still think it's kind of confusing for Duolingo to be throwing in archaic spellings without warning. I suppose it's best for learners to be aware of the older spellings like "daß" for when we're reading pre-1996 texts but right now it feels confusing to be given contradictory information.