If "libAddThis.a
" doesn't have 64-bit support built in, that's why you are currently seeing the "Undefined symbols for architecture x86_64
" error.
You won't be able to support 64-bit in your application, at least until AddThis updates their SDK to support 64-bit iOS.
So for the time being you should:
1) turn off "arm64
" from the "valid architectures" & "architectures" in your app's build settings
2) e-mail the AddThis support people (e.g. via the "Post A Question" link on their iOS SDK page)and tell them to update their SDK to support 64-bit iOS.