Option is greyed out and I cannot enable spell checking in Xcode
I believe this is a bug. I am not sure if it has been fixed. Try make sure you’re on the latest Xcode version.
From Stack Overflow:
I believe this is a duplicate post but I'll answer anyway. This is an Xcode bug. There have been radars filed. As far as I am unaware there is no work around or fix currently.
That was posted at a similar time to when this was posted.
Make sure you are doing it while in a mail composition window, not in message viewer. The spellchecking option is grayed out in message viewer because you can not edit received messages.