There are at least 3 different threads on this bug. Could people please do a search before posting yet another new thread on it again. FYI, even though the button is missing, it still defaults to a "flexible" search for awards. Yes, there are some issues with certain destinations that are only served by partners (although you can still do a day-by-day award search on these by starting with a revenue search).

No more flex date award searches?
Flexible dates not allowed for award tickets?
What happened to flexible date search for miles?