I've been researching all over trying to fix this issue to no avail. I followed the SwiftyDropBox tutorial thoroughly and had everything working. It's now not working and I'm not sure why. Based on this post and this link, the error is supposed to be related to not having the DropBox app installed. That's fine. In the tutorial it says:
If you wish to authenticate via the in-app webview, then set browserAuth to NO. Otherwise, authentication will be done via an external web browser.
I set that parameter to false and still get the same error.
Turns out the issue was that I already had a viewcontroller being presented. It looks like the dropbox authorization acts as a "presentation". It worked once I dismissed the viewcontroller. Those error messages still appear but according to this post it's normal behavior. I wish they'd have more descriptive messages.