This article explains how to fix the virtual keyboard not appearing when in an RDP session for Windows 10

Windows 10 has problems with virtual onscreen keyboards when using an RDP session. Most of the time the OSK will not automatically pop up whenever you are needing to type into a text field. Here is something that may work for you as a work-around.

Go into the Windows 10 OSK settings and change the touch keyboard to "Automatically show the touch keyboard in windowed apps when there's no keyboard attached to your device." See below steps:

1. Click on Start.
2. Open Settings.
3. Select Devices.
4. Go to Typing.
5. Under Touch keyboard settings, switch on Automatically show the touch keyboard in windowed apps when there’s no keyboard attached to your device.