The video below shows how to add a new replicated player stat, for this example i use a float variable called NewStat you can name your whatever you like.

If you want to use an integer instead you can but in the OnRepNewStat function use the UpdateInteger function instead of the UpdateFloat function.

Then search for the RefreshStatUI in the my blueprint panel open the collapsed node and add the nodes in the image below.

You now have a new stat you can set your NewStat variable to whatever value you like by using its set node and the UI will update. For this to work in multiplayer you can only set your stat on the server, if your not familar with how to do this you can look around online for tutorials on replicated events.

The video below is an optional extra it shows you how to create Increase and Decrease functions for your stat that block it from going above the max value and below 0. You can then call these functions from the player inventory to change your stat. For this to work in multiplayer you can only call these functions on the server, if your not familar with how to do this you can look around online for tutorials on replicated events.

If you want this stats value to be saved you can follow the guide linked below.

SGK V2 – Saving new player character data