Trying to save an auto-response and seeing this error?
"This email cannot be converted to use statistics because..."
This usually-large, sometimes ominous-looking, message is actually just a warning. These warnings pop up when using certain merge fields that do not convert over to streaming emails. The warnings are just notifying you that the auto-response cannot be converted to a streaming email since it is using merge fields that are not supported by streaming emails.
Here is an article that explains streaming emails in more detail:
If your goal isn't currently to create a streaming email (as detailed in the above article) and you're just doing something like creating a donation auto-response, then you can just ignore the big warning. Your auto-response will work just as you expect.