A few pointers.
The RB file which makes the extension is put in the base level of the ZIP file.
A folder with the same name is also needed at this base level.
There should be nothing else in the ZIP at the base level - just one RB and the folder with the same name.
All other supporting RB files, images etc go in the folder, or even subfolders within that.
When you make the ZIP file you need your folder-options set to show all file-extensions.
Otherwise the ZIP file that is displayed as my_plugins is not renamed as you expect - when you rename that my_plugin + .rbz it becomes my_plugin.rbz.zip
so it’s not seen by SketchUp…
It needs to have the .RBZ on the very end !
my_plugin.rbz