Omnisharp ships with its own mono version, so, if it is unable to locate the installed one, if you want to tell omnisharp to look for a "global" mono installed in your machine, put this in your settings.json:
Omnisharp ships with its own mono version, so, if it is unable to locate the installed one, if you want to tell omnisharp to look for a "global" mono installed in your machine, put this in your settings.json: