Hey @thimblefoliopress,
You probably have some stray vertices that are causing the issue

See where the lines overlap, then when you join with CTRL+J, Blender will connect those vertices and it kind of twists the line back on itself and Blender has trouble calculating the solid fill.
Delete any vertices so there is no overlap, here I can just delete 1 vertices,
I will probably remove the one on the left, and leave the 2 vertices that will join the ones that are closer together,
and select everything again, then press CTRL+J to join them.
Also, I don’t think there is a major difference with Grease Pencil, but it’s not wise to use any other version than the one used in the course.
Newer versions can be used with some prior knowledge, but Blender is not backward compatible.
Meaning, there could be a new feature in v4.x that was not in v3.x and therefore not replicable.
A change in the first number of the version means a major update, as stated, v3 and v4 have major differences.
The second number change indicates minor updates.
and the third number is basically for bug fixes.
Using v3.4 for a v3.6 course shouldn’t cause to many headaches, but using v3.x for a v4.x is likely to give you nightmares, although as I said I don’t believe there are major changes with Grease Pencil, but I could be wrong.
Can I ask why you have decided to use v3.6 for this course?