Actually, it's not your error, it's ours.
The documentation should not have mentioned an UPDATE action for
tag-values. There was a small change to remove the UPDATE action back in
January (see discussion here http://bit.ly/aaCWHV) and it looks like I
forgot to remove mention of UPDATE from the documentation at
http://doc.fluidinfo.com/fluidDB/permissions.html
That's now been corrected. Sorry for the inconvenience!
You'll notice that the conclusion of that discussion was that we should
probably use WRITE (not CREATE) as the action for writing a tag value. I
didn't make that change back in January, partly due to lack of time and
partly from wanting to minimize changes to the API. We still have an
unresolved (internal) ticket on that one, seeing as everyone agreed it
probably should become WRITE.
Terry