It's because you're missing the colon
exports["resource_Name"]:testExportF("Hello")
Basically when you do table:method(arg), Lua passes along "table" as the first parameter, so it's effectively doing table.method(table, arg)
If you just do exports.resource.function then it's expecting that first parameter. Even though you can do exports.resource_Name.testExportF(nil, "hello")