If you have configured the add-ons step on your subscription product but you see a "kwargs" error when you try to navigate to the add-ons step in the site preview:

Screen_Shot_2017-09-07_at_1.17.02_PM.png

This is because your theme doesn't have the necessary add-ons files in it.

If you quickly published the theme that is showing you this error and then unpublish it (re-publish the theme that was previously live), then the necessary files should be added to your theme.

Now if you preview your unpublished theme and navigate to the add-ons step page then you shouldn't see the "kwargs" error anymore and should be able to see the page.