Though they pronounced ich like "ish," which is not how it has been in Assimil.
"ish" is a regional variation, in various German dialects they pronounce the "ich" laut like an "ish", for example in Sächsisch und Hessisch! I find it strange that a textbook would record someone with this accent, since the official Standarddeutsch pronunciation will be like the "ich" Laut you are used to hearing.
First, if one wants to say "You cannot come today," is it equally correct to say either of the following?
"Du kannst nicht heute kommen."
"Du kannst heute nicht kommen."
I automatically thought it was the latter than suddenly started to doubt myself... I'm actually not sure what the negation rules are. I will have a look tomorrow and edit this post with my findings if I remember, since I'm pretty sure there's something about this in the grammar I bought for university. Just send me a gentle reminder if you don't find an answer to this before!
I also massively approve of your decision to learn German! I love this language, there is so much wonderful literature written in German and I think the language itself sounds wunderbar.