"唯一的致命的弱点" can be translated into English as "the only fatal flaw" or "the sole critical weakness", depending on the context and the desired nuance.
"The only fatal flaw" emphasizes that this is the single, decisive defect that could lead to failure or disaster.
"The sole critical weakness" highlights that this is the one and only significant shortcoming in a particular area or context.
Both expressions convey the idea of a singular, crucial deficiency.