OK, you get a traceback telling you line whatever, prop not found, but when you look at that line, you can't seem to find any property there. If that line has anything beginning with a $, (as in $string_utils, $rpg, $player, etc), whatever has a $ in front of it is a reference to a property on #0 (aka The System Object). When something is "@corified" on a moo, a property is added to #0, so when you see something like $string_utils, that means that you can also find:
Owner: Wizard (#2)
So if you can't see an obvious property that is being referenced, see if the "prop" referred to is a corified object.
If the object in question doesn't have any kids, just @chmod object.prop !c
If the object has kids, there are several things that can help, but the easiest, and best way to handle it is to use a verb on $wiz_utils. Your fix would look something like the following:
;$wiz_utils:set_property_flags(#object, "property-name", "r")
Note that in the eval that you use, you DO want to have the name of the property and the permission bits you want, enclosed in double quotes as written above. For more info on this useful verb, type: 'help $wiz_utils:set_property_flags'.
(BTW, no, you don't need to be a wizard to use this!)
First thing to check - ALL verbs on an FO need to be +x. (#1 reason why FO's don't work is the programmer forgot to set the verbs +x)