I got bit by this recently. Not all fare classes are covered for immediate upgrade. According to
https://www.aa.com/i18n/aadvantage-p...a-airlines.jsp
"Y, B, H, K, M, L, V, S or N fares after ticket purchase; all other fares at 72 hours before departure"
If you are booking on alaskaair.com search with "mvp gold" and you'll get the appropriate fare class to get an immediate choice (well, after you book the ticket).