If I select my plan during the SEP but forget to pay, resulting in cancellation of my enrollment, can I enroll again if I’m still within my 60-day window?
If you enroll and then don't pay your premium and get cancelled you don't qualify for special enrollment. One of the only events that doesn't qualify you for special enrollment if failure to pay. This prevents people from chaining together non-payments and special enrollment periods, thus essentially creating a year long open enrollment. Of course this isn't what you were trying to do so you can attempt to appeal this with the Marketplace or your insurer.