Can't install WTG on my external SSD

[admin]

I have a Lenovo laptop that unfortunately only came with a 32GB onboard NAND flash drive which is HOPELESSLY too small for Windows 10 and MS Office. It also has no SATA port on the motherboard so I have resorted to getting a 120gb Western Digital SSD drive and putting it into a USB3 external drive enclosure and I want to install Windows to it.

I have created a a single NTFS partition on the drive, formatted it and assigned a drive letter. I then tried to run the WinToUSB using my Windows.iso file created yesterday using the Microsoft Media Creation Tool. However, when I click Proceed, it tell me to install a USB drive and hit Refresh.

I don't know what I'm doing wrong.

I've successfully managed to do a WTG installation on my 32GB USB flash drive but this SSD won't work.

Any help would be appreciated.

Yo said 'However, when I click Proceed, it tell me to install a USB drive and hit Refresh.'. I'm sorry, I don’t quite understand what you said. Could you explain it? 
Thanks.