If Zinnia fails to create a new journal on your command, we have a couple of ways to approach the problem.
The first, and most likely, solution is to restore your purchase. If the app can't identify your active subscription, it might refuse to add journals if you've previously created more than three. Restoring the purchase will correct this.
Beyond this, in other instances in which Zinnia doesn't create new journals as it should, rebooting your device generally resolves the issue. Simply restart your device and check if you can create a new journal after it restarts.