Instead of:
You could write:
#set($docu = $xwiki.getDocument($docuSpace))
#if (!$doc.isNew())
## The doc exists
...
The reason it fails is because you’re passing a Document object to exists() but it accepts a String or a DocumentReference…
So if you want to use it, you’d write:
#set($docExists = $xwiki.exists($docuSpace))
But better use what I’ve shown above, it’s more efficient.