How do I install OS in Andriod phone without using fastboot and TWRP?
Here is the Full Forum discussion link to the problem I've faced. Thanks everyone who tried to help me out. 

For Quick Solution follow this steps mentioned below:


If you still have access to TWRP on your device? please push 32bit flashable ROM to your device.
Try with a TWRP flashable like the stock ROM Oreo flashable https://forum.xda-developers.com/mot...-test-t3873367 and follow the instructions to install it (i.e. push to your device, or copy to an SD card, then tap install in TWRP to flash it to your device). This TWRP flashable should work on this mentioned TWRP version since it's 32 bit.
The stock ROM download would still require fastboot, so I'm not sure why the fastboot is not working and not choosing the fastboot method as of now.


FYI: error 255 when flashing custom Oreo or Pie ROMs means that the TWRP is 32 bit, whereas the custom ROM requires a 64 bit TWRP.