"Undefined symbols for architecture"
In the General tab, make sure that Placenote.framework is added to Embedded binaries. Also check that ARKit is added to Linked Binaries
"Built for unsupported file format"
Check that you have the Placenote binary in your project. In your project folder, open Assets > Placenote > Plugins > iOS and check the size of Placenote.framework. The framework folder should exist and should be > 6MB. If it's smaller, you didn't download the right library. Make sure you download the .unitypackage file and not clone the github.
"Placenote.framework/Placenote' does not contain bitcode"
Go to your Build Settings tab in XCode and check that Enable Bitcode (it's under Build Options) is set to NO
Check that Privacy - Camera Usage Description and *Privacy - Location Usage Description" fields exist in the Info.plist under the Info tab.
Make sure you're building to a device. You cannot build ARKit apps to an XCode simulator.