One aspect of a sovereign state is that the government of that state controls all the territory within the state and holds a monopoly on the "legitimate use of force" within the state. That is, only the government has the right to use armed force within the country in most circumstances. These conditions did not exist under feudalism. Feudalism, therefore, had to end before states could be truly sovereign.
In a feudal society, all sorts of lords beneath the level of the monarch have legal control over their small territories. They rule those areas and have the right to use force within them. In theory, they are subordinate to the monarch, but that is not always how it is in practice. Often, some lords get to be so powerful that monarchs cannot control them and do not effectively rule the land where those lords hold sway.
The end of feudalism puts an end to that system. It makes it much easier for the central government to rule every bit of land in the country because it takes power away from these lords who can compete with monarchs for control. In this way, the end of feudalism helps to allow the creation of sovereign states.