I've had this problem dozens of times while using Xamarin it is by far the most annoying thing. It just slowed me down by 2 hours trying to get one id to be detected. I tried all the usual things.
Clean, Build.
Clean, restart vs, rebuild.
Delete resource.designer.cs in combination with cleaning and rebuilding.
I did eventually get the Id to show up on the resource designer, but still, the same error.
This time I couldn't take the hit to my productivity any longer and instead changed the id to an existing one.
But this is ridiculous and needs to be fixed my Microsoft ASAP. Why does this always happen? How hard is it to get the resource file properly generated and detected by VS? Please, this is the number one pain point with Android development on Xamarin. We should be able to make our own ids without all these issues.