Out of Blizzard's official SystemCheck Troubleshooting FAQ:
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One last troubleshooting step to try is to change the language on your Battle.net account to something different, and then switch it back to the correct language. This step resolves some extremely rare "jams" in the system. To access your language settings please log into Battle.net; choose the "Account" option at the top of the screen; choose "Settings"; choose "Communication Preferences", and then use the Language drop-down menu to toggle your language selection. Push "Submit" at the bottom of the screen to finalize your language change, and then repeat these steps again to switch it back to your preferred choice. Afterward, try submitting the beta opt-in again.
If you are still having a problem feel free to contact our Technical Support team.