I took a look at your capx.
I couldn't get the demo to do anything, but....
Create a variable: "temp_Build_UID"
Event 37
When a player clicks a "build object", add a piece of code:
set global variable >> "temp_build_UID" to build.UID
// this lets the game remember which build tower was clicked last.
Event 38
then, in the event where the tower is built, add another condition
pick the Build object by UID >> then use the temp_build_UID variable
// this has the game pick the build tower that was last clicked
In the same event, add a piece of code:
destroy "build object."
// this destroys the last clicked build object.
hope that helps!