When attempting to create a new email from a Marketo Guided template and editing the code to add styling for your brand, the modules on the right disappear.
Editing the code of an email created from a Guided template will break the email's relationship with the template. The recommendation is to either add all of the modules you want and then edit the styling, or create your own custom template with the styling for your brand already in place. See these articles for more information on how to do this:
NOTE: In order to pursue creating your own template, you will need to have a developer available. Marketo support is not equipped to troubleshoot custom coding.