The North Koreans have been printing U.S. currency for decades. The newer anti-counterfeiting seems to have addressed the problem. Any nation state has the capability to do this so Ukraine’s counterfeiting of the Russian ruble will be answered in kind.
The interesting and operative question here is does the security level of the Russian Rouble lend itself to easy counterfeiting? Is it anywhere near the current US level for our currency?
At some cost to Americans. If you own any of the older $100 bills you may find banks or currency exchange outlets unwilling to accept them.