This validation fires in two situations:

• You are creating a new kitchen with Is Active? on while the number of active kitchens is already equal to (or above) your Max Active Kitchen Locations limit.
• You are editing a kitchen with Is Active? on while the number of active kitchens is above the limit (for example after your plan was downgraded).

Either deactivate another kitchen first (untick Is Active? and save), save the new kitchen as inactive, or purchase an additional kitchen slot through Click here to buy on the list page. Other save errors you may see are "Name empty" (the Name field is required) and "Update Error" (a system problem while saving — retry or contact support).