First, understand what the RAND Corporation is. It is a thunk tank whose clients are composed of the US military and other State actors. The RAND Corp's job is to provide their "clients" with arguments and background info in support of furthering the U.S. national agenda.

Naturally, the U.S. saw Nord Stream as a threat, because it would allow Germany to become more friendly with Russia, and would provide Germany with a direct pipeline from Russia - without having to rely on the Ukrainian pipelines. This was important to the Germans, because - surprise, surprise - Ukraine is a corrupt country, and there was a LOT of stealing going on, with Ukraine skimming gas for themselves and overcharging the customers in Western Europe.

Don't forget also that in 2019 Ukraine was firmly and deeply a client state of the U.S., and was "buying" and/or "borrowing" lots of money, weapons and other material from the U.S. - the Americans did not want to see Ukraine's primary source of foreign income be endangered by a Russo-German pipeline.

Secondly, Nord Stream would have provided energy to Germany and Europe that was as cheap as the energy in the U.S. itself, thereby giving the Europeans a chance to compete even more effectively with American industry.

So, for the Americans, implementing the sanctions and then blowing up Nord Stream was a way to kill several birds with one stone. It helped the U.S, and its client Ukraine, and at the same time, it hurt both Russia and America's economic rivals in Europe.