Public matches in Nettla require a minimum number of participants in order to go ahead. If a public match is created too close to the scheduled time, there may not be enough time to reach this minimum — which is why the option to create a public match may not be available in the very near future in the mobile app.
The threshold for how far in advance a public match can be created is controlled by your club's automatic cancellation rules. You can find these settings in:
Settings → Rules & Limits → Automatic cancellation rules (at the bottom of the page)
The key rule to look at is "If only 1 player has joined the match" — the number of hours set here determines how far in advance a public match can be created. For example, if this rule is set to 24 hours, a public match can only be created if it is scheduled at least 24 hours from now.