ipad mini 2 activation locked

Good evening to all! I have recently fixed up an old iPad I bought from a lady online which I put my own apple id to use app store. Now, after an update, the iPad locked itself up and displays activation lock. The seller is trying to remove it but it cannot be removed. I have a spare iPad mini 2 16GB over this 32 which I transplanted it's parts over. The iPad is also not on lost mode. How to fix this issue? It runs iOS 12.5.7 I think. Please help as the iCloud removal tool is also saying the serial is invalid.