Go to the BP_InGameStartMenu and create a new variable called ShowSaveButtonIfClient and set its type to Boolean and tick Instance Editable.

Create another new variable called ShowLoadButtonIfClient and set its type to Boolean and tick Instance Editable.

Add the highlighted nodes shown in the image below, location at the top of the image.

If you want the save and load buttons to be hidden for clients then leave the variables default values unticked, if you want them to be visible for clients then tick both variables default values.