linker command code failed with exit 1

When I try to build, I keep coming up with this error message: Undefined symbols for architecture x86_64: "_ACAccountTypeIdentifierTwitter", referenced from: -[PF_Twitter _getLocalTwitterAccountAsync] in Parse(PF_Twitter.o) "_OBJC_CLASS_$_ACAccountStore", referenced from: objc-class-ref in Parse(PF_Twitter.o) "_OBJC_CLASS_$_SLComposeViewController", referenced from: objc-class-ref in Parse(PF_Twitter.o) "_OBJC_CLASS_$_SLRequest", referenced from: objc-class-ref in Parse(PF_Twitter.o) "_SLServiceTypeTwitter", referenced from: -[PF_Twitter _getAccessTokenForReverseAuthAsync:localTwitterAccount:] in Parse(PF_Twitter.o) -[PF_Twitter _getLocalTwitterAccountAsync] in Parse(PF_Twitter.o) ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation)