When you first start with Piano, you may encounter an error with your offer templates that prevents them from rendering when you attempt to preview them:
This is actually not an issue with the Piano preview. Piano's offer templates work by pulling in data from an existing offer to render the content. If you do not have an offer with at least one term, Piano cannot pull any content in to preview. As a result, you will see this error, which is identical to what users would see if you were to use the Show offer card to display an offer that did not contain any terms.
If you do see this error screen, creating a term and an offer that contains it should cause it to go away.