I have been able to run charm through the terminal and use the SimNIBS gui without an issue. Unfortunately, it looks like the issue is with matlab not being able to recognise charm.
I don't know why the "system" command executed from Brainstorm creates a different environment than the terminal from which you start SimNIBS directly. It should be similar...
Make sure there is no difference of enviornment in the terminal from which your start Matlab and the one from which you start Matlab.
In the meantime, you can run SimNIBS manually and import the output in Brainstorm with the menu "Import anatomy folder" (similarly to FreeSurfer, but selecting the file format "SimNIBS").
I would greatly appreciate if you let me know whether there are updates regarding charm and brianstorm or the FEM computations.
If you want to get updates, you can "watch" the GitHub repository (GitHub - brainstorm-tools/brainstorm3: Brainstorm software: MEG, EEG, fNIRS, ECoG, sEEG and electrophysiology) and/or configure your forum account to receive all the post.