Two weeks ago I received your letter, but I could not answer you until now.

not 是否定词，用于构成否定句，表示"不"或"没有"。 until 是介词，表示"直到...为止"，强调某个动作或状态持续到某个时间点才发生变化。not...until 是一个常用搭配，表示"直到...才..."。