invalid reference image arkit 1.5

As in Apple's documentation

For each image, use the inspector to describe the physical size of the image as you’d expect to find it in the user’s real-world environment, and optionally include a descriptive name for your own use.

Click on your image and chose size inspector to provide size. :)

In my particular case, the dimensions were set and the image was named. But the name itself was invalid because I had another file (of a different type) named the same.

If the accepted solution cannot solve your issue, try changing the AR Resource's name.