[ns] ask about the tclsh

[BEILU SHAO]

dear all,
 when i implement the ns-simple.tcl, it says the following:
"When configured, ns found the right version of tclsh in
/home/linux21/shao/ns-allinone-2.28/bin/tclsh8.4
but it doesn't seem to be there anymore, so ns will fall back on running
the first tclsh in your path. The wrong version of tclsh may break the
test suites. Reconfigure and rebuild ns if this is a problem."
 what is my problem? 
many thanks!