I loaded a blueprint file and BookyAI says "This file is not valid JSON". How do I find the mistake?

From version 1.9.22, BookyAI says exactly what is wrong and where: the line and the column (as your text editor numbers them), the line itself with a marker at the spot, and how to fix it, for example a missing comma, a comma after the last entry, or a single quote where a double quote belongs. Open the file in a plain text editor (not a word processor), go to that line, fix it, save, and load the file again. Curly quotes added by a word processor are straightened by BookyAI itself. Fields BookyAI does not read are named too, with the one you probably meant ("did you mean summary?"), and sections that are not part of the blueprint format go into the Codex as notes.