Go to Settings > About Phone and tap Build Number 7 times.
What (e.g., touch not working, bootloop, 0MB storage) made you look for a repack? What Android version is your Vivo Y71 currently running? Do you already have the stock firmware file downloaded? twrp vivo y71 repack
After these are enabled, you would typically connect the phone to a PC with ADB and Fastboot tools and use the fastboot oem unlock command. However, the exact command can vary, and in some cases, Vivo's bootloader may require a specific key or not be unlockable at all using standard methods. Go to Settings > About Phone and tap Build Number 7 times