Typical awful Marriott nonsense on the website with zero regard for formatting.

Stay Longer and Save, 30 percent off Stay 3 and Save, prepay in full, non-refundable if cancelled less than 1 day before arrival, based upon availability

What that's saying is you're booking the "Stay Longer and Save" rate which is 30% off the regular rate. You need to "stay 3" (minimum nights) and save. The reservation is prepaid in full and will become fully non-refundable if cancelled less than 1 day before arrival (that's 11:59pm day before in hotel time). Meaning no charge up front but they will charge the entire booked stay at the 1 day mark if you haven't cancelled. "Based on availability" is utterly meaningless here and should never be displayed. The fact that the rate is appearing means it's available.

-RM