Here
on join:
while player is online:
set title of player's scoreboard to "Scoreboard title"
set line 1 of player's scoreboard to "&7Gems: &6%{money::%player's uuid%}%"
wait 1 second
This requires SkBee
Also important info for you
You wrote the variable like this
"%{money::player's uuid}%"
Which is "wrong" if you want player based variables because the variable currently is only read like you read it here.
The correct way would be
"%{money::%player's uuid%}%"
That way the player's uuid will be used instead of just the word "player's uuid"
Here's an overview how it looks for skript
"%{money::player's uuid}%" -> {money::player's uuid}
"{money::%player's uuid%}%" -> {money::aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee}