Administrations tend to put up a page at whitehouse.com on issue X when they want to trumpet or otherwise refer to what the administration has DONE about issue X. Since Trump just got elected, he hasn't done anything noteworthy on climate, so there's no page describing what he's done - speeches he's given, bills he's signed, etcetera. Since there's no such page, there's no such topic. Simple as that.
The topics there now seems to be cut-and-paste from his campaign site; it's essentially a placeholder to be filled out later.