I was able to install the extension you linked in both 1.2.40 and 1.3-beta21.
However, I took a look at your Lua script and it doesn’t seem to be prepared as an extension. It looks like it would be more suitable to register a new command that would run this code when clicked on a menu option.
But the script is run by clicking “File → Scripts → 8BPExporter” then you get the txt file.
Ok, regarding the aforementioned errors, I think it is because of the following:
I use aseprite portable and I think you use aseprite installed.
I think the scripts are only installed if the user has aseprite installed.
The scripts will not be installed in the portable version of aseprite.