hmm this is a tricky one…one solution would be to recompile it yourself. OwenAshurst.com
the other solution would be to wait for an OSMC representative to answer this thread cause maybe they know a way…
From this thread here: No ffmpeg available
i see that there is an alternative…but like i said…i am not sure…better wait for someone who knows this…worst case scenario you reinstall osmc…if you cant wait and want to ‘play’ with it 