Due to a bug from Google picker, sometimes you'll see 'No documents' when trying to select your Drive folder to send mail merges with personalized attachments. Read this article to know how to solve this issue.
Why do I get 'No documents' when trying to select my Drive folder?
1. I double-clicked on the Drive folder that contains my attachments
2. I opened my Drive folder from the search bar results
How to solve this Drive folder selection issue?
1. Select your Drive folder directly from the list
If your folder is visible in the list, simply click it only once (do not open it). Then click Select.
2. Click 'Search for all items containing...' in the search bar result
Instead of clicking on the result, you'll have to either click the link Search for all items containing... (or press Enter).