understanding a new command when the thing is not present

I’m trying to do something which I thought should be pretty basic in Inform. Yet it doesn’t work. Inform requires the “thing” to be present. How can I write a new command when the noun ISN’T going to be present? If I want to summon a demon… same problem coding the command.

sandcastle is a thing.
Understand the command “build” as something new.
Understand “build [thing]” as building.
Building is an action applying to one thing.
Check building:
if the noun is the sandcastle:
move the sandcastle to the location of the player;
say “A magical [noun] appears before your eyes.”;
say “You don’t have the skill to build that.”.

Two steps: make it apply to “one visible thing” (the terminology is a confusing relic of the early days of Inform; “visible” here means “not necessarily touchable”), and change the “build [thing]” line to “build [any thing]”.

Though I’d suggest making a new type for things you can build, and restricting the grammar to that.

Also, if I may…
Don’t forget to include ‘instead’ with your ‘say’ phrases, under the Check rule.