Go to the PlayerBuildingComponent and create a new custom event and call it ServerUpdateReference and set it to Run on Server with Reliable tick on.

Next create another custom event and call it ClientUpdateReference and set it to Run on Client and Reliable ticked on. Then add a new input and set the variable to BP_BuildMaster and call the input BuildMaster.

Then copy the images below by adding the highlighted nodes, locations at the tops of the screenshots.