I'm using G++ 13 on OpenSuse, this's my version. Make sure you've the same version or above.
First of all, compile iostream to a module.
g++ -fmodules-ts -x c++-system-header iostream
If you see this folder, everything worked!
Now, you can use the code here! Just run:
g++ -fmodules-ts sum.cpp main.cpp -o sum && ./sum
If you see the number 3 in your terminal, everything worked!