The `MINIMAL_VIEWPORTS` is no longer contain 'iphone6' and 'iphonex' in [code](4f5ab9fe9e/addons/viewport/src/defaults.ts). In the meantime add `import { INITIAL_VIEWPORTS } from '@storybook/addon-viewport';` for more clear example
4f5ab9fe9e/addons/viewport/src/defaults.ts