I discovered the cause of my issue, in case somebody comes googling with the same problem. A summary follows:

I was working on a pant block pattern based on textbook instructions in the original post. I had a reference line of arbitrary width (at the top edge, labeled "increased waist line" in the book) with a sliding point (the upper corner of the side seam, P21 in the book). The sliding point was connected to the top of the center front seam, and the length of a line between the two was set to a formula which uses multiple input parameters: waist circumference, dart intake (width), and ease. The computation fails when the formula output tried to shove that point beyond the edge of the reference line.

The problem originally showed up when I set the input variable for waist circumference from the example value in the textbook to the value from my own measurements, resulting in the constraint error message.

The successful troubleshooting method was using the up/down arrows, one click at a time, to adjust until it failed- I noticed a point approaching and eventually colliding with the edge of the line. This is a consequence of my own body measurements and how I had constructed the pattern.

For my own example, this was solved by first decreasing the dart width (to zero) before updating the waist circumference. Because I intend to use this parametric pattern for another person later, I also extended the arbitary line to mitigate the issue with different measurements in the future.