Final answer:
Capulet's servant complains because he's illiterate and cannot read the list of invitees for the ball, which creates a humorous situation and helps advance the plot by leading him to Romeo.
Step-by-step explanation:
In Act 1 of William Shakespeare's Romeo and Juliet, Capulet's servant complains about his errand to invite people to a ball because he cannot read. The list of invitees is written down, but the servant is illiterate, which makes it impossible for him to know who to invite without seeking help. This predicament adds a comedic element to the play and also introduces the servant to Romeo, which leads to Romeo and his friends learning about the ball and deciding to attend it. This, in turn, sets up the crucial first meeting between Romeo and Juliet.