If the scheduling panel says "No scheduling round is open", Roll Call opens one within a couple of hours. Click Reschedule there to start one now.
If it says "Next round waits", no date is open for the next round. The sentence names the cause and where to fix it, and Roll Call opens the round once a date opens.
Otherwise, when a scheduling round returns no options, one of six things is usually the cause. Check them in this order.
Once only you can fix it, your dashboard label reads Needs You.
1. Quorum is too high
Each extra player required can cut the number of workable times.
Lower the quorum by one, if it is above 2.
2. Windows are shorter than a session
When Roll Call builds a poll, it leaves out any day whose session schedule window is shorter than a session. Shorten the session, or widen the hours on at least one day.
3. Players have not responded
A player who has not answered the session poll cannot count toward quorum.
While players are still answering, the panel says it is waiting for them and adds the one fix that helps, such as a nudge or a lower quorum.
The campaign page shows who is outstanding. Players who connect a calendar and turn on Auto-RSVP answer automatically.
4. Dates are blocked
Blackout dates and automatic holiday skipping can remove more of a period than expected. Check the campaign's blocked dates for the range you are scheduling into.
5. No eligible period
A campaign with a planned end date may have no dates left before it. Move the end date in campaign settings.
Once the campaign has nothing left to schedule, the campaign page offers Extend.
6. Your own time off or calendar
Roll Call also removes times you cannot make as the GM: your time off, your connected calendar, and any game you are already running. If your own availability is tight for the period you are scheduling into, it can empty a round on its own. Check your time off and calendar on My Availability, or widen the campaign's days.
Still nothing
If none of these apply, contact support with the campaign name.