Hi Chris, first you've got an extra parenthesis after days and also you need to escape the enclosed backslashes (or it's working without any enclosed parenthesis). An easy way to insert the search in your document is with the "New Search" feature when you right click in the sidebar where the Searches are. If you want to embed it in your document, just tick the "Save as embedded @search in document" when creating the new search.
Here is your corrected search:
Next 7 days @search(\(@due < [d] + 7 day\) and not @done)