It looks like you have declared a page in pages and classes of the section, that why its showing up in the autocomplete. No worries since you have already created the HTML rule it should be picked up in the Runtime. Just Keep the HTML stream name in that textbox.
Please Click on the Button and Observe if the pdf is getting generated or its coming blank.
I have checked the log file, I see there is no error message on the step pages, Also your section mark up (HTML Stream) is correctly populated and passed to HTMLtoPDF activity, PFB below screenshot. Can you please check in your Pega logs, what exception is being thrown and attach here.