Cant figure out how customize plugin

I'm trying extend the EmmyLua Plugin functionalities, Lua 5.4 has a 'Power Patch' that becomes with Compound Operators and others many(small) features.

The plugin repo issues seems abandoned, so I'm trying do it by myself. As a new and inexperienced developer, i'm getting stucked at something that I cant figure out.

I want use '+=' and '-=' without catch an exception. After editing some files, I cant make it work. Could someone do a review and tell what i'm doing wrong?
There is what I did: