。
Don't forget to lock the door.
Literal
Not-forget-[without-ず] door [possessive-の] key [object-を] apply-[soft-command-なさい]
鍵をかける is the standard idiom for 'to lock': literally 'apply a key'. The possessive construction ドアの鍵 = 'the door's key/lock' is typical for parts of things. かける itself is a highly polysemous verb ('hang', 'apply', 'spend', 'sit', etc.) — learning its collocations is a project of its own.