Press "Next" in the shipping step of the checkout and wait for the payment step. When the form refuses (a required field is empty) the step ends on the shipping step: follow it with a field error assertion.
Examples
When I continue to the payment step
And I continue to the payment step
When we continue to the payment step
When continue to the payment step
And we continue to the payment step
Pattern
When(/^(I |we )*continue to the payment step$/, …)
Part of @webship-js/magento-steps (github.com/webship/magento-steps), a step pack for Webship-js 2.0.x.