'lectio' is not a chapter, but a reading/lession. If chapter is what is intended here, please use 'capitulum'.
There's no source I can find that gives "lectiō, lectiōnis" as "chapter."
In fact, there are many good sources that suggest it should be "caput, capitis."
"capitulum, capitulī" could also work, though what I've read is that this is Late Latin
It is Ablative.
You aren't going to see the Locative really. It comes up a LOT in the early stages, but by Classical Latin it fell out; it is also always without a pronoun.
Here you're looking at the Ablative of Place Where, which absorbed the Locative.
This course is in Beta, meaning it is understood that some tweaking is required to get the flow of teaching better ordered.
"In urbe" and "domi" were introduced very early, as 'set phrases', to make the sentences more interesting from the start.
This phrase is in the same vein, but where the basic theme is to introduce, and to get students confident in the recognition of and the use of the Accusative, there does seem to be a lot of casual dropping in of the Dative and Ablative, before they have been introduced and discussed.
Personally, I try never to socialize with cases to whom I have not been given a formal introduction. ;)