Why did Congress tell President Lincoln that he was not allowed to sign the official copy of the Thirteenth Amendment?

Lincoln signed 14 copies as a symbol of his enthusiasm and support for the resolution. Soon after, the Senate declared Lincoln's signature "unnecessary," rebuking him on the grounds that the president has no constitutional role in proposing or passing constitutional amendments.

Explanation:

(don't quote me)

Lincoln signed 14 copies as a symbol of his enthusiasm and support for the resolution. Soon after, the Senate declared Lincoln's signature "unnecessary," rebuking him on the grounds that the president has no constitutional role in proposing or passing constitutional amendments.