Add the is valid node in the image below, location at the top of the screenshot.

Next move the Switch Has Authority to the location shown in the image below.

Go to the BP_InventorySaveSystem bp then to the Save Master Grids function, you then need to replace the Add array nodes with the Add Unique node like show in the video below. You need to do this for each Add node and set the grid type to the same of the Add node you are replacing.