The checkbox list in "Progress" in the PRs body says:

"Commit message must refer to an issue"

but it is not really talking about the commit message, but the PR title. The "refer to an issue" is also vague; especially since the only time you will be hit by this problem is when you are a newcomer, and do not understand what is expected of you.

The text should be rephrased to be more helpful to newcomers.