Resolve build tools error,Unable to resolve build tool - Unity

This is my first time building anything with unity and When I try to save the project as a apk file, I'm getting the "unable to resolve build tools directory" error. I've downloaded and switched the Android root folder with googles sdk and also downloaded the ndk r13b but I'm still having no luck. I am using the free version of Unity. Don't know if that makes a difference. Any help is MUCH appreciated!