i need a little help about Replication in my game

since few months i start making my own game following multiple tutorials but i have one little issue i followed a tutorial Talk about making fortnite building System but the project was only for one player not multiple player in the server so when i build a wall or floor it show only for the local player and i want it to show for others so can someone give me some help plz
can some one take a look and tell me how to fix this problem and thanks
the tutorial playlist