When did Rubio make this promise? The deal you are talking about happen in 1994.
In 1994, Ukraine agreed to transfer these weapons to Russia for dismantlement and became a party to the Treaty on the Non-Proliferation of Nuclear Weapons, in exchange for economic compensation and assurances from Russia, the United States and United Kingdom to respect the Ukrainian independence and sovereignty
Marco Rubio was not even a government official until 2000.