Can not find type or namespace for Rift to Quest conversion program

I am trying to convert a program for the Oculus Rift to Oculus Quest.  I wrote this a couple of years ago in Unity 5.5.0f3.  I am now using Unity 2019.1.8f1.  After some tinkering, I am down to 3 errors.  The first of these errors is:  Assets\OVR\Editor\OVRLayerAttributeEditor.cs(25,30): error CS0246: The type or namespace name 'OVRLayerAttribute' could not be found (are you missing a using directive or an assembly reference?).  Thanks in advance for any advice.  I have been away from Unity and programming for a while, so if you could include menu paths and other details, that would be really appreciated!  ;)