I had something very similar happen to me today, when porting the Environment chapter of UPBE (the same error in a figure reference, where one line would work, but a similar one (or, actually an exact same one that I retyped) line would compile fine).
It turned out that, somehow, there were non-printable characters in the file name.
You can see if this is the case for you by either doing a 'cat -v' of your file, or just re-typing the line yourself.
I have no idea how those weird characters made it into there, by the way! Maybe something exotic with cut&paste, or a find&replace with regexes in Atom.
Actually, I just pulled a fresh copy of the repo, and it doesn't look like there's trouble characters.
But, I've noticed a couple of other things:
2) All of the labels for the figures are the same, 'obj'. The labels are used for in-line figure references, so having all of the same labels is not helpful (I'm surprised the compiler doesn't complain).
3) Most importantly (this is what actually fixed the error and allowed the book to compile), you also have to add the new chapter to support/templates/book.latex.template, in the graphicspath section. (It can't find the file because it doesn't know to search PharoLauncherTutorial/figures/). (This is mentioned in
https://github.com/SmalltalkZen/UpdatedPharoByExample/blob/master/CONTRIBUTING.md in the 'adding a chapter' section).
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