Looks like the installed hg.exe is not in the path. Sorry I don't use Windows, so I don't have insight to recommend the next step. Mozilla matrix https://chat.mozilla.org has a #introduction channel for helping with basic developing of Mozilla. There must be some developers using Windows that might be able to help. https://wiki.mozilla.org/Matrix#New_to_Matrix.2C_new_to_Mozilla.3F
Bug 1885695 Comment 7 Edit History
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Looks like the installed hg.exe is not in the path. Sorry I don't use Windows, so I don't have insight to recommend the next step. Mozilla matrix https://chat.mozilla.org has a #introduction channel for helping with basic developing for Mozilla. There must be some developers using Windows that might be able to help. https://wiki.mozilla.org/Matrix#New_to_Matrix.2C_new_to_Mozilla.3F