how do i give effect when a player picks up(in hand) a sword(custom sword) ?

Here is some base code to work with on "picking up" an item.  You can look for attributes on the item picked up, and apply effects as you like.

 

Spoiler

@SubscribeEvent
    public void onItemPickup(PlayerEvent.ItemPickupEvent event) {
        ItemStack itemStack = event.getStack();
        String itemNamespace = "";
        String itemPath = "";
        Item item = itemStack.getItem(); //Somehow translates to the item name (minecraft:sand is "sand" here)
        try {
            itemNamespace = Objects.requireNonNull(item.getRegistryName()).getNamespace();
            itemPath = Objects.requireNonNull(item.getRegistryName()).getPath();
        } catch (Exception e) {
            LOGGER.info("Unable to get item namespace or path.");
        }
        String itemId = itemNamespace + ":" + itemPath;
        ServerPlayerEntity player = (ServerPlayerEntity) event.getPlayer();
        String playerName = player.getName().getUnformattedComponentText();  //Just player name.

        Minecraft mc = Minecraft.getInstance();
        if(mc.player != null) {
            //mc.player.sendChatMessage("[" + playerName + "][" + player.getPlayerIP() + "] picked up [" + itemId + "][" + itemStack.getCount() + "]");
            LOGGER.info("[" + playerName + "] picked up [" + itemId + "]");
        }
    }

 

 

first you mix Server and Client Side code,
since you cast the Player of the Event to a ServerPlayerEntity which is Server-Side
and then you use the Minecraft class which is completely Client Side
you should use a instanceof check bevor like:

PlayerEntity player = event.getPlayer();
if (player instanceof ServerPlayerEntity) {
  // do server stuff here
} else if (player instanceof ClientPlayerEntity) {
  // do client stuff here
}

if you cleaned up your code (which you should definitely do),
you could use PlayerEntity#addEffect