TWRP recovery installed but not opening I have Vivo y15s , I rooted my phone with fastboot (oem unlock)
I installed TWRP recovery.img using fastboot 
But 
My phone is not starting and TWRP recovery mode is not opening.
Only Fast Boot mode is working..
My suspicion is that you might have flashed the wrong TWRP image. TWRP images are specific per device. Look for your device here:
https://twrp.me/Devices/

If you find the correct one, then reflash stock OS, then oem unlock, then flash new recovery.