Unlock the wizard by saving or discarding changes in the save bar.
In the New Task wizard, Next Step can stay greyed out for two reasons.
Unsaved changes
When you change a setting, Shopify's save bar appears at the top of the admin. The wizard shows You have unsaved changes. Please save or discard your changes to continue to the next step. Click Save or Discard in the bar, and the button becomes active again.
Nothing selected
On Choose Specific Items, you must select at least one product or collection. The wizard reminds you with Select at least one product to continue or Select at least one collection to continue.
If the button still does not respond, reload the app; your draft can be restored with Continue Draft.