I cannot use some of the machine learning modules without enabling my discrete Nvidia GPU.
To save power and reduce noise, I disable the discrete graphics card (using the Nvidia prime-select tool) .
In this mode, running NetTrain leads to the error:
LibraryFunction::libload: The function WL_MXGetVersion was not loaded from the file /usr/local/Wolfram/Mathematica/11.3/SystemFiles/Components/MXNetLink/LibraryResources/Linux-x86-64/MXNetLink.so.
Re-enabling the Nvidia graphics card and rebooting fixes this problem. However, the default mode of NetTrain is to run with 'TargetDevice -> "CPU" ' so surely I should not need to enable my Nvidia card to get this to work.