you need the book to be NOT in mobi format in order to easily edit it.
if you have it in epub you could look for the character that is not working, and search/find/replace it for one that does work, after conversion. it's a bit trial & error until to get the hang of it.
spot it with your current file IF you convert that to back epub with calibre & then load it into sigil for editing, but if the damage is already done you need a better quality source.
switch to nook & all your mobi problems will go away
meanwhile you test by loading .epub into (free) adobe Digital Editions on PC - if it looks OK there AND looks ok in calibre viewer, THEN it will probably be OK on Nook also