Error when using Slug field

[dakala]

I've got this code:

Flexible::make('Practitioner types', 'practitioner_types')
            ->addLayout('Practitioner type', 'practitioner_type', [
                Text::make('Label', 'label'),
                Slug::make('Slug')->from('Label')
            ])
            ->collapsed()
            ->button('Add practitioner type'),

The slug field doesn't work and I get an error. See the screenshot please:

What is the problem and how do I fix it please? Does it matter that I'm trying to do this with https://github.com/outl1ne/nova-settings ?