how to set item property at runtime

Hi

I want to set item property runtime,

so far i've used following code to set property

but runtime i received error that Invalid Item

app_item_property.set_property('<block_name.field_name>',ENABLED,PROPERTY_OFF);  

so i used following code

app_item_property.set_property(find_item('<block_name.field_name>'),ENABLED,PROPERTY_OFF);

but same error persists.
 

Thanks,

Shrinath