Still no solution to this? i have the same problem

 

If anyone happens to come across this, there are multiple solutions but here is a common one:

You need a computer running macOS (or Mac OS X), Linux, or Windows 7 and up. I managed to do this on my Hackintosh macOS Mojave so it should work with any version running at least Mavericks.

Download ADB, configure it, and download and install LG L90 D415's respective ADB AND Fastboot drivers. Reboot your computer if necessary.
Download the TWRP image for the L90 (codename w7): https://dl.twrp.me/w7/

Plug in your L90.
Make sure it's in Download mode.

Type in the following commands in a command prompt or terminal window. Ignore anything after the ///. Comments so you can understand what does what. Elevated privileges are usually not necessary:

Code (Text):

1.  
2. adb reboot bootloader /// reboots to download mode. If you're already in this mode, don't worry about this cmd.
3. fastboot devices /// Checks if your device is visible. If you do not see your device, you may need to install the drivers again. Try rebooting after installing them.
4. fastboot oem unlock /// Unlocks bootloader. If you did not have a developer option to turn this on or off, it will allow you to unlock it regardless.
5. fastboot flash recovery recovery.img /// READ: This is the command that flashes TWRP. Replace "recovery.img" with what ever the recovery name is if it did change its name. Make sure you typed it correctly!
6.  

Assuming all commands went through successfully, you can reboot. To get into TWRP, unplug your device and make sure it is not turned on (Important!). Hold the Volume Down button and the Power Button. While holding those two buttons, plug in your device simultaneously. You should boot into TWRP now!
If you did not boot into TWRP, try the same button combination except hold Volume Up instead of Down.

Now to flash a custom ROM and install Magisk, mount your device via MTP or USB Storage, download their respective flash-able ZIP files and move them onto the device, dismount the device, wipe ALL partitions EXCEPT for Micro SD and USB OTG, and then install the ZIPs with the ROM first!

If you somehow cannot manage to get USB Storage and MTP mounted onto your computer: get a microSD card, plug it into the computer, download the flash-able ZIPs onto the microSD, dismount it, and insert it into the phone and reboot into TWRP again to detect the SD card. I had to do this with my ZTE Zmax Z970.

Hope this helped anyone who finds it!