Upfare UA flight - missing the catch?
So have an upcoming UA SAN-DEN flight in L class. A few days ago, cash upgrade opportunity to FC (P) was $139, I look today to book and now there is no cash upgrade available, only for points/waitlist. The flight is still 28 days out, and avail classes were F9, A9, JN9, C9, D9, Z6, ZN6, P0, PN0, R0, RN0, IN0, I0, Y9, YN9, B9, M9, E9, U9, H9, etc (9 down the line).
What's the best advice to do a cash upgrade, as the option to doesn't show and didn't want to spend miles. What am I missing on the details and what advice would you give?