Jesus said "As my Father has sent Me, so I am sending you". This was the commission of the Son. So, at that time, the Son had "all authority in heaven and on earth".
"Jesus said, "All authority has been given to Me in heaven and on earth."
The next sentence...
"Go therefore..."
Contextually Jesus is claiming: "I have the right to commission, empower, and send you... just as my Father commissioned, empowered and sent me."
That was not the Old Testament covenant. This was the New Covenant ratified by the death and resurrection of the Son who had been given the all authority from the Father.
I wonder why this is difficult for you to see.