IANACE (I am not a Construct expert), but...
The clean way is probably to split your events onto multiple event sheets and use "event sheet includes" to use only the events you want.
https://www.construct.net/en/make-games/manuals/construct-3/project-primitives/events/includes
Another way, if you really want to have only one layout sheet for simplicity or some other reason, would be to use some unique feature of your layout to make conditional events fire. You could check the name of the layout, for example.
But the point is on first level (layout), the character won't have any weapon but until second level the player able to pick up weapon and use it, or is there any way to include character having weapon start from level two (but the event sheet must same as level one)