Because when I use an addEvent () calling a local function that is inside the same script returns error. Is that right?
Nope. addEvent works fine with local functions.

https://github.com/otland/forgottenserver/blob/master/data/globalevents/scripts/serversave.lua
You might be passing a userdata object. In that case the error message should indicate that.
Please use the issue template in the future and include the scripts that are required to reproduce the issue as well as the error message you have received.
Most helpful comment
Nope. addEvent works fine with local functions.
https://github.com/otland/forgottenserver/blob/master/data/globalevents/scripts/serversave.lua