Search for "TWRP Vivo Y71 1726" on trusted developer forums like XDA Developers. Download the .img file and move it to your ADB installation folder on your PC.
fastboot boot twrp_vivo_y71_full.img
Understanding your hardware is critical before attempting to flash custom recovery files. Vivo Y71 (PD1731F) Chipset: Qualcomm Snapdragon 425 (MSM8917) CPU: Quad-core 1.4 GHz Cortex-A53 OS: Funtouch 4.0 based on Android 8.1 Oreo twrp vivo y71 full