the sim no longer asking for puk but complain unsable simcard

Hi Adebola, you have 10 chances to enter correct PUK, if you exhaust the 10 chances, your SIM will be damaged, you will have to do SIM swap.
To swap your MTN line,

You need to visit any MTN service centers or connect stores nearest to you with the following:
Phone Number,
NIN Number,
Your full name, date of birth, state of origin and mother’s maiden name.
4 Regularly dialed numbers within 30 days.
Last recharge date and last recharge amount
A valid identity card (Driver’s License, Voters card, National ID card, NIN slip document, International Passport)
SIM Pack or Damaged SIM with Serial (for damage SIM only), or other evidence of direct purchase/ownership of the SIM
Cost is N400 but you might be required to also buy airtime.

You can dial *123*7*1# to get information on the location of any of their shop closer to you.
When you dial the code, you will be required to enter your State, then your Local Government Area, you will receive SMS on the nearest MTN office address in your location, or you can use this link: https://www.mtnonline.com/personal/find-a-store/