You need to specify the pvob which references all UCM objects, including projects:
cleartool lsproject -invob \yourPVob
# Or, on Unix
cleartool lsproject -invob /vobs/yourPVob
If you want just the names, you can take advantage of some of the options of cleartool lsproject
:
cleartool lsproject -s -invob \yourPVob
Make sure you have a 7.1.x recent enough version of ClearCase: this IBM technote reports the cleartool lsproject
command don't work well in 7.0.1.7 and 7.1.1.0.