Once you choose a template that utilizes dynamic content and/or personalization tags in your campaign you must always edit the campaign in the HTML Editor; you cannot change to the Email Editor.
Search Existing HTML Templates
Navigate to Campaigns > Create > Template.
Select your property from the Property drop-down list. Select Responsive Templates from the Category drop-down list and Legacy HTML from the Editor drop-down list. Select Search.
Preview a Template
Click the Preview link within any thumbnail to open a full-size preview of that template.
Select a Template
Click a thumbnail to select it for your campaign. Select Save & Continue.