Why does my plain text version say "Your message does not contain a plain text alternative"?

This particular Preview shows how clients that don't have HTML email support would show your email, or how your email would appear if a user had disabled HTML emails in their client.

If you've used the paste your HTML option within Litmus, be sure to include a plain text version in the corresponding tab in order to test it. If you've sent in an email you should be sure to include a plain text version as an alternative part to your email, otherwise plain text email clients may show an empty email or your HTML code as plain text.

For further information on how to test a plain text version of your email, check out this help article.