Thats the same Problem with me I could not choose a Deck or a see a card choosing a deck was difficult. I asked the Pokemon Company this is what they told me(I was using Windows 7) : Hello *****,
Thank you for contacting Pokémon Support! We understand you are experiencing an issue accessing the TCG Online Cliet.
We recommend that you uninstall the client from your computer and then re-install the client again with a secure internet connection. We recommend that you uninstall the game and all of it's folders from your computer. Then, when you are re-installing the client, install the TCG Online files into your Program Files (x86) folder. After doing this please attempt to run the TC again and have it update itself.
We apologize for the inconvenience and thank you for your patience and support in this exciting time! If you have anymore questions or comments about this process please feel free to update this ticket with them.
Pokémon Support Team