Context: The Paris Peace Conference, 1919 gathered 27 nations at the Palace of Versailles to shape the future after World War I. The Russian SFSR was not invited to attend, having already concluded a peace treaty with the Central Powers in the spring of 1918. The Central Powers-Austria-Hungary, Germany, Bulgaria and the Ottoman Empire-were not allowed to attend the conference until after the details of all the peace treaties had been elaborated and agreed upon.

Question: who was not invited to the treaty of versailles?
Steam of consciousness below:
The relevant information to answer the above question is: The Russian SFSR was not invited to attend, having already concluded a peace treaty with the Central Powers in the spring of 1918.
So, the answer is The Russian SFSR.