Thank you for the quick response Amy, it was not following through to the Schedule M. Ultimately, the system was not allowing me to allocate a proportion of the contributions to the state level (I was a part-year resident) for some reason. I made a change to allocate a portion of my federal interest income (unrelated to the 529 but part of Schedule M), and then it allowed me to allocate the 529 contribution.
A note for anyone else who experiences this issue, it does appear to be a glitch in the system, so one possible workaround could be to enter in $1 in federal treasury interest, allocate it to Illinois, and then you should be able to allocate the 529. Just make sure to go back and delete the treasury interest after.