【I know no such person _____ you have mentioned.】為何此句空格中不能填 that ?

I know no such person as you have mentioned.=
I know no such person as the one that you have mentioned.
the one that宜省略，以免成為贅詞。但我們不能只寫that，而不用as。
such as是在舉例用的。

通常that關係代名詞用於事及物，人則使用who,whom.
此題是mention的受詞，故應使用whom.

I know no such person who(或 "that" 代替) you have mentioned.
#(文法書) 關係代名詞用於主格而指「(先行詞) 人」時用 "who"，指「(先行詞) 人、事、物、動物」時用 "that"。
#(英辭典) that：釋義5. [relative pronoun] Used instead of ‘which’, ‘who’, ‘whom’, or ‘when’ to introduce a defining clause, especially one essential to identification.

例1. ‘the woman that owns the place’
例2. ‘the book that I've just written’
例3. ‘the year that Anna was born’

such--adj no comparative or superative, not placed between the indefinitive article(a) and its noun.
I know no such (a) person (as) you have mentioned.
"that" adj pron, used when the person is near to the speaker
=I know that;
=who/whom should be used.
eg:-I know no such person "whom" you have mentioned.