BuildFailedException Reference Library missing texture at index 1

Hello
I'm following this video on YouTube at my first attempt for image tracking on Unity in AR, but for some reason I encountered two things that the video did not:

First, at minute 8:57 he used Debug.Log(trackedImage.name); while I had to use Unity.Debug.Log(trackedImage.name); for it to work on Unity.

Then when I finished everything else step by step I had a BuildFailedException such as the following:

Code (CSharp):

1. BuildFailedException: ARCore Image Library Generation: Reference library at 'Assets/ReferenceImageLibrary.asset' is missing a texture at index 1.
2. UnityEditor.XR.ARCore.ARCorePreprocessBuild.BuildImageTrackingAssets () (at Library/PackageCache/com.unity.xr.arcore@2.1.8/Editor/ARCoreBuildProcessor.cs:286)
3. UnityEditor.XR.ARCore.ARCorePreprocessBuild.OnPreprocessBuild (UnityEditor.Build.Reporting.BuildReport report) (at Library/PackageCache/com.unity.xr.arcore@2.1.8/Editor/ARCoreBuildProcessor.cs:33)
4. UnityEditor.Build.BuildPipelineInterfaces+<>c__DisplayClass15_0.<OnBuildPreProcess>b__1 (UnityEditor.Build.IPreprocessBuildWithReport bpp) (at <af78ad00a40d4f61b82afc6cafb0a416>:0)
5. UnityEditor.Build.BuildPipelineInterfaces.InvokeCallbackInterfacesPair[T1,T2] (System.Collections.Generic.List`1[T] oneInterfaces, System.Action`1[T] invocationOne, System.Collections.Generic.List`1[T] twoInterfaces, System.Action`1[T] invocationTwo, System.Boolean exitOnFailure) (at <af78ad00a40d4f61b82afc6cafb0a416>:0)
6. UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
7.  

Are the errors related? And how can I fix the errors? Thanks.