If you are using a standard repack in a .zip or .xml format, follow these steps: Vivo Y12i Firmware Repack [better]
Most MediaTek and older Qualcomm phones allow you to flash raw rawprogram0.xml files. The Vivo Y12i, however, uses a proprietary verification system. The stock firmware downloaded from the internet often lacks the specific or has partition tables that don't match your exact hardware revision. vivo y12i firmware repack