Android BTLE -> Cannot find callback wrapper

While you can, as the excepted answer says, safely ignore this message, it is telling you something. Most likely it is telling you that your device has location permissions turned off for your app which is preventing your beacon scanning from working. If the app has proper permissions and is properly scanning you will see something more like

D/BluetoothAdapter: STATE_ON
D/BluetoothLeScanner: onClientRegistered() - status=0 clientIf=5

If you are targeting API level 23+, even if your minsdk is set lower (like 16), you definitely want to check in your code for location permissions being enabled.

I lost many hours to this before realizing it was just silent failing due to location services being turned off for the app.

Be careful not to take lines you see in LogCat too seriously if they are not from your own code. This is especially true if the line starts with "D/", which indicates a debug line, not an error ("E/") I have built dozens of beacon apps, have seen that line more times than I can count, and can confidently say that it does not indicate a problem.

Bottom line: you can safely ignore that message. Be aware that the Android Bluetooth stack is notoriously noisy in LogCat. This will not be the first line you learn to ignore.