Activation Lock: Not supported or valid

Hi! I’ve recently been interested to get back in to my old iPad gen 3. My Apple ID and password were both written on my case of my iPad. But the passwords doesn’t seem to be working. Where it says Not support or not valid. I’ve tried so many things as well as go into Apple support or apple activation lock. Which almost helped at first but sadly because it’s a old iPad, we no longer have the original receipt. Which it asks for but we do have the serial number and what kind of model it is. What should I do?