I have been told that the that of a cleft construction is not a pronoun at all. I honestly don’t know what to call it. No, you cannot use it there.
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AnonymousYou may use a comma as you'd normally pause between early and on while reading out loud, think of it as...= "it was his relative lack of sucess early on (his career, his life, etc.), that ultimately".There is no reason at all to use any additional commas in the sentence. Doing so would be poor punctuation.
Son JamesIronically,
Son JamesCan the word "early on" be an adverbial phrase?Yes, it is adverbial.