Mercury (Budh) is the planet of communication, intellect, and the voice. The 1st house (Lagna) represents the self and the physical body.
When Mercury sits in the 1st house, the person is often blessed with immense artistic talent. They have a natural affinity for music and can play instruments ("saaj") beautifully. However, Lal Kitab gives a very specific and strange warning: They should never sing professionally.
If a person with Mercury in the 1st house tries to become a singer, their voice (Mercury) clashes with the ego (1st house), leading to public humiliation, loss of wealth, and ultimate ruin ("barbadi"). Their talent lies in their hands (playing instruments), not in their vocal cords.