I ran into the same problem a few weeks ago. I had to upgrade my Parallels 8 (to get Windows 10 working), but when I clicked to upgrade it turns out Parallels 11 was out so I couldn't get Parallels 10 upgrade anymore. I asked ParallelCares and they said they no longer sell Parallels 10 and I have to buy Parallels 11. Kinda sad, I bought Parallels 6, paid to upgrade to Parallels 7, paid to upgrade to Parallels 8, but I'm reluctant to pay full price now to get Parallels 11. Basically it's like I'm starting out as a new purchaser of virtualization software. I love Parallels, but paying $80 to get windows 10 working on my machine vs $45 with VMWare Fusion (Parallels upgrade/switch to VMWare) just didn't make sense to me. I hope you have better luck with ParallelCares. In the end for me it works out better too since we use VMWare at work.