I'm pretty sure I know the answer to this, but it's worth pointing out and I'd be interested in the reasoning behind the changes. In all previous OS X versions, it's been possible to use setxattr() (as root) on /Library/Application Support. This seems to be no longer supported under Sierra, giving a "not permitted" error. It is also no longer possible to even "sudo chmod a+w" on Application Support (same error).
Can someone explain if this is in fact a behavior change in Sierra? I can't find any relevant documentation to back this up.
This is almost certainly the result of enhancements to System Integrity Protection Guide (SIP). If you turn off SIP do things work?
Why are you trying to set extended attributes on a system directory?
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