[CrazY.]

is_user_alive already check if the player is connected, so you don't need to verify. Also, you need to check if weapon is a valid entity.

public SkinCheck(id)
{
    if(!is_user_alive(id))
        return PLUGIN_HANDLED;
    
    new Sil, WPName[32];
    formatex(WPName, charsmax(WPName), DefaultModels[get_user_weapon(id)][1]);
    new WPN = find_ent_by_owner(-1, DefaultModels[get_user_weapon(id)][1], id);
    
    if (!is_valid_ent(WPN))
        return PLUGIN_HANDLED;

    if(get_user_weapon(id) == CSW_M4A1 || get_user_weapon(id) == CSW_USP)
        Sil = cs_get_weapon_silen(WPN);
        
    if(get_user_weapon(id) == CSW_AWP)
        cs_set_user_zoom(id, 0, 0);
    
    PlayAnimation(id, AnimationIDs[get_user_weapon(id)][Sil]);

    return PLUGIN_HANDLED;
}

Also, consider to use switch instead of a lot of "if"