Once a competition (League, Tournament) is created, you can then assign teams to compete in that competition. Until that has been done, you cannot move forward.
Matches are also directly linked to competitions. Once you have at least two teams assigned to a competition, you can then create a match between those two teams.
Why is it important I create a competition?
Without establishing a competition, you lack the framework necessary for creating matches between competing teams. It's essential for at least two teams to belong to the same competition before a match involving those teams can be initiated