I have been trying for 3 day to opt in to Shadowlands beta on the Shadowlands launch page. Keep getting an error that it was not able to complete the opt in at this time. I also can’t find the beta opt in page on my blizzard account where you can toggle all blizzard game for beta.
One thing to try is clearing out your browser’s cookies and cache and reload the browser. If possible try a different browser altogether.
Unfortunately, the only way to opt-in now is via the Shadowlands page. The section that used to be in account admin on the site was removed a while back.
Here is the page: Shadowlands Beta Opt-In