Is it possible to read local variable throungh OPC communication?

Hi Kang,
The local variable's scope is within the process in which they were created.
https://gcsresource.aveva.com/Citect/WebHelp/citect2018R2/Content/Local_Variables.html
They are not exposed outside the process that creates the variable.
You may want to change them with Persistent one and then they can be exposed via OPC.

According to your words, local variables cannot communicate externally, and in order to communicate externally, should the local variable be registered in the variable tag using the disk device?

I am pointing to a different technology.
If you want your local variable to be exposed outside you need to have them managed by the IOServer and this means that you need either define it as part of an IO-DiskDev (quite old technology) or part of a device that uses Persist Memory (a caching mechanism) similar to disk device but more efficient.