Solved Getting this error and I don't know how to fix it:

E_ERROR - Call to a member function sendMessage() on string

here is the plugin code:

PHP:

<?php

namespace Hittmana\TradePro;

use pocketmine\command\CommandSender;
use pocketmine\command\Command;
use pocketmine\plugin\PluginBase;
use pocketmine\Player;
use pocketmine\event\Listener;
use pocketmine\event\TranslationContainer;
use pocketmine\utils\TextFormat;

class MainClass extends PluginBase implements Listener
{
    public function onEnable()
    {
        $this->getLogger()->info("Test enabled");
        $this->getServer()->getPluginManager()->registerEvents($this, $this);
    }
    public function onDisable()
    {
        $this->getLogger()->info("Test disabled");
    }
public function onCommand(CommandSender $sender, Command $command, $label, array $args)
{
    if($sender instanceof Player)
    {
         $name = $sender->getName();
         if ($command->getName() == "test")
         {
            $this->getLogger()->info("IT WORKS!!" . $name . "USED IT!!!!");
            $recipient = $args[0];
            $trader = $sender->getName();
            $recipient->sendMessage($trader . " wants to trade with you!");  
            $sender->sendMessage("You sent a trade request " . $recipient . ". Lets hope he says YES!");
            return true;
    }
    else
    {
    $this->getLogger()->info("Command must be run in-game or by a player :(");
    return false;
   }
  }
}
}

You aren't checking if $recipient is a player.

oh ok thanks! so I use

PHP:

if($sender instanceof Player){ 
other code
} 

To check correct?

I said recipient, not sender.

PHP:

if(!$recipient instanceof Player){
 $sender->sendMessage("Player not found");
}
else{
 //code
}

ok thanks! Just out of curiosity because I'm new why is there a ! before $recipient ?

That means NOT

Ok thanks so much for the help!

Mhm

$recipent is $args[0], $args[0] is a part of a typed, command, means that args is a string, so you need to do $recipent = $this->getServer()->getPlayer($args[0]);

Args isn't a string. $args is an array, which you get the first(counting starts from 0 at programming) entry from: $args[0] and the entry is a string. If you want $args as a string, you have to use implode().

Ok now it doesn't find a player at all!
Here is the code:

PHP:

<?php

namespace Hittmana\TradePro;

use pocketmine\command\CommandSender;
use pocketmine\command\Command;
use pocketmine\plugin\PluginBase;
use pocketmine\Player;
use pocketmine\event\Listener;
use pocketmine\event\TranslationContainer;
use pocketmine\utils\TextFormat;

class MainClass extends PluginBase implements Listener
{
    public function onEnable()
    {
        $this->getLogger()->info("Test enabled");
        $this->getServer()->getPluginManager()->registerEvents($this, $this);
    }
    public function onDisable()
    {
        $this->getLogger()->info("Test disabled");
    }
public function onCommand(CommandSender $sender, Command $command, $label, array $args)
{
    if($sender instanceof Player)
    {
         $name = $sender->getName();
         if ($command->getName() == "test")
         {
            $this->getLogger()->info("IT WORKS!!" . $name . "USED IT!!!!");
          
            $recipient = $args[0];
             if($recipient instanceof Player)
                 {
             $trader = $sender->getName();
            $recipient->sendMessage($trader . " wants to trade with you!");  
            $sender->sendMessage("You sent a trade request " . $recipient . ". Lets hope he says YES!");
            return true; 
            }
            else
            {
            $sender->sendMessage("Player not found");
            }
    }
    else
    {
    $this->getLogger()->info("Command must be run in-game or by a player :(");
    return false;
   }
  }
}
}

How do i fix the plugin not working?

… Look at the error!!!
Call to member fubction on string]

Not that thats fixed

nvm

$args[0] is the player name that the command sender typed. It has absolutely nothing to do with a player object. Just from a logical sense, not even talking about API knowledge, there is no way that PHP/PocketMine/whoever would know that that certain command argument would be related to a player. (Say, maybe it may be a world name, a plugin name, or whatever? Who knows?)