PUK stands for PIN UNLOCK KEY. You get this error when you wrongly entered SIM card pin for 3 attempts.
The PUK is not available anywhere online as this is unique to your sim card. The only one who can provide this to you is the network provider. Just call up their customer service line and then ask how you can obtain the PUK.
the puk code can be provided by the customer service or you career by calling them from another sim card and provide them your phone number that you had blocked