This is always a problem with a new pistol. One suggestion, remove the magazine and look in the well with a flashlight. You can see the nub of the tool in its locked position. Try putting a drop of oil or spray a little silicone on the nub and then try removing the tool. The only real reason to remove the tool is to change the backstrap, as the takedown lever can be moved with your finger, pocket knife, etc.

Do not turn it more than 1/4 turn or you will break the nub off the shaft. I broke off the nub on my M&P soon after I got the gun. I made a call to S&W customer service, told them what I had done, and they sent me a new one free.