A similar error occurred slightly earlier in the month, which was later rectified. So hopefully the same issue has occurred this time as well. It was mentioned on one of the announcements that the Beta is available to purchase up until launch day. Hopefully one of the friendly staff can jump in and shed some light on this soon.
As per the FAQ pinned in this forum, it is still available: Automobilista 2 FAQ Can I buy Automobilista 2 Beta? AMS2 Beta is currently available exclusively to backers from the Automobilista 2 Early Backing Campaign - the campaign will be closed on February 29th. AMS2 Beta will continue to be a part of the Bundle including AMS2 & all planned DLCS until AMS2 is released on March 31st - AMS2 Beta will not be available for new users after Automobilista 2 v1.0 is officially released.