Missing serial number for installation

Hi all. In 2018 I bought a physical copy of Photoshop Elements 2018. I installed it and stuck the disk in a drawer. I have recently had to run a factory reset on my PC and am now in the process of trying to re-install PSE2018. I put the disk in, went through the installation process, but am stuck at the final hurdle: I cannot find the product's serial number. I have read everything on the box and the disk sleeve and found nothing which could be a serial number. I checked online and it said the serial number should be on the sleeve of the Windows disk. Both my Mac and Windows installation disks were in the same sleeve, and that sleeve does not contain a serial number.

Since I was able to install the product 2 years ago, I can only assume that I had the serial number then, but have subsequently lost it. I have moved house since then and there is no chance of finding it again if that is the case.

Does anyone know how I would go about being able to find or get a new serial number to activate the product? I have proof of purchase, both in the form of the box/sleeve and in the form of the emailed receipt from Amazon. In the current economic climate I cannot afford to buy a new copy. 

Thank you in advance.