I can't use my license key for Lightroom 6.

Hi, I have a Lightroom 6 CD with license bought 5 years ago from a photo shop in Prague. I have reinstalled the program recently but is telling me the license key is already in use. I was the only one using this license key. What should I do? I can send photos with the actual CD and envelope with the license key to prove I'm the real owner.

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