An Assembly can have different statuses: open or building.
1/ Building Assembly
These Assemblies are not yet ready to organise collections. They are either missing Members, Producers or haven't found a suitable venue yet.
That's the reason why you can't place an order with those Assemblies yet. But you can help to open them by asking the Host(s) if they need help with something specific or simply spreading the word!
2/ Open Assembly
These Assemblies have regular collections. Generally, Assemblies hold the collections once a week or every other week.
Each online market lasts 6-7 days and finishes the day before the collection in the evening, to allow Producers enough time to organise themselves.
They are thus no permanent online market. You will be notified by email when a new online market opens (unless you have unticked the "my notifications" box) and current open markets are shown at the top of the homepage.