Visible Light Communication (VLC) has emerged as an innovative networking technique post-5G, leveraging its expansive license-free spectrum. However, the main challenge remains the narrow modulation bandwidth of light sources. To address this, Non-Orthogonal Multiple Access (NOMA) has been proposed as a successful method to significantly enhance spectrum efficiency in downlink VLC systems. This study evaluates the error performance of a NOMA-based downlink VLC system using high-order modulation schemes, specifically M-Quadrature Amplitude Modulation (M-QAM). MATLAB simulations are employed to assess the Symbol Error Rate (SER) for two users. The study investigates how changing the modulation order impacts SER for both users and examines the influence of Power Allocation (PA) coefficients. Results indicate that increasing the modulation order affects SER differently for each user. Moreover, adjusting the PA coefficient for the farther user can improve performance at higher signal to noise ratio (SNR) but shows limited effectiveness at lower SNRs. Also our work investigates downlink NOMA-Based VLC System, concentrating on how different modulation techniques affect the system performance.