Hello,
Welcome to Microsoft Community.
The Windows 10 10240 version is the initial release of Windows 10. As far as I know, the firmware driver of Surface Dock2 at the time of release is already version 17763 and the minimum version is 17763.If you need to use Surface Dock2, it is recommended that you upgrade the Surface Laptop5 to Windows 10 to a newer version.
If anything is unclear, please feel free to let me know.Thank you for your patience and understanding!
Reagrds,
Manson | Microsoft Community Support Specialist