Ok, I see, the issue is that the PR must be rebased before being merged but the PR wasn't created with option allowing committers of the main projects to rebase on the forks.
It's confusing here as it is your fork, so we may want to check if the user is allowed to rebase on the project the PR is coming from to allow rebasing via the UI regardless of the status of the allow_rebase option.

I am ignorant, where do I find the allow_rebase option? Sorry, you wrote it; it is per-PR option.

Commit c5ff01d fixes this issue

It's a checkbox when you open a new PR :)