Hey there,
Start from scratch. Make sure you installed the processor and graphics card correctly. Attach the graphics card into slot PCIEX16(G5). For testing purposes, mount one RAM module into slot A_2. Install one NVME SSD into slot M2_2 or conventional SSD using an SATA cable. Plug in all power connectors on the motherboard [ATX_POWER, ATX_12V_1, ATX_12V_2]. Insert an HDMI cable into the back port of the motherboard and one for the graphics card. Fire up the computer. Let us know what's happening.