This happens if the partition structure of the new firmware is different. Change the dropdown in SP Flash Tool from "Download Only" to "Firmware Upgrade."
Install the MediaTek VCOM drivers on your PC. You may need to disable "Driver Signature Enforcement" in Windows 10/11 to do this successfully.
Try a different USB cable or a different port (USB 2.0 is more stable than 3.0 for flashing). Ensure the VCOM drivers appear in your Device Manager.