Sorry for my reply (I do not use Mac ... and so - I not able to check it with F-Secure XFENCE);
How I can to understand your steps was next:
--> You opened specific XFENCE beta-program link and tried to use button "Register" (as fresh registration for F-Secure beta-portal);
--> When you type there your mail-address and enter - it will create such situation (as your screenshot);
Does it valid for your experience?
If yes - do you able to try next option-steps:
----> Just open main beta-portal page as: https://beta.f-secure.com/welcome/
----> Register there your account (with your mail) as "create fresh account";
----> Login with your credentials to beta.f-secure.com portal and check if there available XFENCE beta-program and choose it. Or from XFENCE promo beta-page (with your fresh credentials) directly;
If no - sorry for my suggestions (and if you already have beta account; so maybe you able to try latest step as check if there available beta-program XFENCE with beta-portal and your account);
Sorry for my reply. I think that such situation anyway should be fixed. So, will be good if there F-Secure Team did some response.