So, I Made a System for Choosing a Weapon When You Spawn. It Works Fine in The Core Editor but When I Play the Game It Doesn't Spawn in at all
It's not easy to diagnose as you don't provide much information, but I'll take a guess that when you say it works in Core editor you mean for shift + play, which has you testing on a merged server + client version of your game in a single window. If you just press the play button you should get a client window(s) and the editor scene should show the server content. If you publish unlisted you'll get a full test of what the players will see.
From this I expect that your problem is that you are only spawning your weapon on the server. You could try enabling networking on your spawn script, but without seeing the script and the hierarchy structure it's hard to know if that would be enough. If you want a simple spawner to copy, go to Community Content and import Weapon Spawner by WaveParadigm. You just need to add a weapon template to it: WeaponSpawn (networked) - Custom - Weapon and you are good to go. At the very least this will show you a networking setup that works.