As announced few years ago and with the move from Skype for Business Online to Teams, the Skype for Business Online administration portal will be retired starting December 1st, 2020

Access to this administration portal (which you should probably did not access for quite a while anyway).

Also as reminder the Skype for Business Online PowerShell module (used to connect to SfBO) has been merged with the Teams PowerShell module (see https://t.co/xmiV2ezIjg)