Every server includes one backup slot. The slot count is the number of backups the server can hold at once — not a limit on how often you can back up.
When all slots are full, delete or unlock an old backup before taking a new one. Locked backups cannot be deleted by accident, so a full set of locked backups will block new ones until you unlock one.
To increase the limit: the slot count is part of your plan, not a setting inside the control panel. Open your service in the Billing area, choose the upgrade option, and pick a larger Backup Management option — 1, 2, 3, 5 or 10 slots are available. The change applies to your existing server; nothing is reinstalled and no data is lost.
Backups are stored separately from your server, so they survive a reinstall — but they are not a substitute for keeping your own copy of anything you cannot lose.
If you need help with this, please submit a ticket.