Because, “You have received it” is an independent clause introduced by the relative pronoun ‘that’ (If "that" is a relative pronoun, there must be a noun which is referred to by "that". If it is a noun clause, "that" is called "noun clause marker".) It does the function of a noun and a noun can be an object complement, as explained below:
You can call him a scoundrel.
The word "scoundrel" is an object complement
that describes the direct object "him
You can call him what you wish.
The noun clause "what you wish" can take the place
of "scoundrel" as object complement.
I have already known the funcitions of an object complement very well.
According to you explanation, there is no noun which is refered to by "what" before "what you wish". That shows "what you wish" is surely a noun clause.
However, in the sentence we're discussing, there is a noun (a confirmation) which is refered to by "that" before "that you have received it". That doesn't show "that you have received it" is surely a noun clause.
Noun clauses perform the same functions in the sentence which nouns do, I have already known, but I have never heard it can function as an objective complement.
Have a good time!