尝试连接braintree时出现问题

Question

我已按照此处所述添加了所有内容：braintree drop in并执行pod安装。但我得到了下一个错误：

Undefined symbols for architecture arm64:
  "_OBJC_CLASS_$_BTThreeDSecureRequest", referenced from:
      objc-class-ref in libBraintreeDropIn.a(BTCardFormViewController.o)
  "_OBJC_CLASS_$_BTPaymentFlowDriver", referenced from:
      objc-class-ref in libBraintreeDropIn.a(BTCardFormViewController.o)
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

任何人都可以帮助解决链接器错误 - 可以做些什么？