Mario I'm currently experiencing the same issue, yet it's not ignorable - following the tutorial I'm not able to import either the .skel-json file, nor the .skel file for either method. The file icon is X'd out and I just get "Error importing 'res://Art/Output/skeleton.skel'." messages. I can't just proceed as it gives me errors if I try use the file. If I drag the skel file to the Skeleton file res, it says:
"No loader found for resource: res://Art/Output/skeleton.skel (expected type: )"
If I try load it through the path it says:
"Cannot load resource from path 'res://Art/Output/skeleton.skel'"
As a result, the skeleton file res just stays <empty> as godot just wont let me input the file at all to it. currently, there's no way to continue past this if the error occurs, as far as I can see.