I'm using Quicken Classic Deluxe v8.3.3 on MacOS Tahoe.
I have several scheduled bills set up in Bills & Income. Right now, the only way I know I have upcoming bills is if I go to the Home screen, Bills & Income screen, or the related account's register.
Several years ago I distinctly remember having pop-ups announcing I have a bill payment due. The reminder would appear prominently, regardless of what Quicken screen I happened to be looking at. In other words, if I had a credit card payment due Quicken would notify me without having to go looking for the reminder.
I looked in the help topics and can only find instructions on how to enter a new bill reminder but there is no mention of an option to see it in a pop-up