Your automatic cancellation rule for "only 1 player has joined" sets how far ahead a public match must be created. Inside that window, players can't create public matches or split-payment private matches, because they would be cancelled before they could fill.
Example
If Cancel if only 1 player has joined is set to 6 hours, players can only create a public match that starts at least 6 hours from now. For a match starting sooner, they can still make a court booking or a private match paid in full.
Where to change it
Go to Settings → Rules & Limits.
Open the Auto-Cancellation tab.
Under Match Cancellation Thresholds, change Cancel if only 1 player has joined, with this much time remaining:.
Click Save.
Good to know
The same window applies to split-payment private matches. Private matches paid in full are never auto-cancelled, so they can be created at any time.
Shortening the window lets players create matches closer to the start, but gives them less time to fill.
You can create matches in the calendar inside this window only if they won't be auto-cancelled straight away (for example, by adding players).