Fix actually using the mod

src/main/java/net/brokenmoon/afloydironchest/mixin/MixinEntityPlayerSP.java

@@ -1,5 +1,6 @@
 package net.brokenmoon.afloydironchest.mixin;
 
+import net.brokenmoon.afloydironchest.MixinInterfaces.IEntityPlayerSP;
 import net.brokenmoon.afloydironchest.gui.GuiDiamondChest;
 import net.brokenmoon.afloydironchest.gui.GuiIronChest;
 import net.minecraft.client.Minecraft;
@@ -8,7 +9,7 @@ import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.Shadow;
 
 @Mixin(value = EntityPlayerSP.class, remap = false)
-public class MixinEntityPlayerSP {
+public class MixinEntityPlayerSP implements IEntityPlayerSP {
     @Shadow
     protected Minecraft mc;
     public void displayGUIIronChest(IInventory iinventory) {

src/main/resources/fabric.mod.json

@@ -32,7 +32,8 @@
   },
   "custom": {
     "loom:injected_interfaces": {
-      "net/minecraft/src/EntityPlayerSP": ["net/brokenmoon/afloydironchest/MixinInterfaces/IEntityPlayerSP"]
+      "net/minecraft/src/EntityPlayerSP": ["net/brokenmoon/afloydironchest/MixinInterfaces/IEntityPlayerSP"],
+      "net/minecraft/src/EntityPlayerMP": ["net/brokenmoon/afloydironchest/MixinInterfaces/IEntityPlayerMP"]
     }
   }
 }