Hello yo,
well, to answer your question, the reason you're getting the "no such variable" error, is because AOLserver is expecting a variable named "conn" and it can't find it. Normally you would create this variable in your tcl program by saying something like:
set conn "some value"
However, I don't think that's really going to solve your problem as I can see you are using the "ns_conn form" command and (as far as I know) this doesn't take any parameters (see the documentation on http://www.panoptic.com/wiki/aolserver/192)
What is it you are trying to do? I normally never use ns_conn because OpenACS has plenty of higher level functions for accessing the form variables (e.g ad_page_contract or template::form and also ad_form). If I were you I would search for existing examples of those ways of programming and then copy them.
hope this helps
Brian