What did Jeremiah protest when God called him to be a prophet?

Answer: "I am only a child!"

Jeremiah said he did not know how to speak, "for I am only a child!" God replied, "Do not say, 'I am only a child.'"

““Ah, Lord GOD,” I said, “I surely do not know how to speak, for I am only a child!” But the LORD told me: “Do not say, ‘I am only a child.’ For to everyone I send you, you must go, and all that I command you, you must speak.”— Jeremiah 1:6-7 (BSB)