Google Analytics announced they have finally completely sunsetted the old version of Google Analytics.
You can now only use the new UI (user interface) for Google Analytics. This is a really long time coming.
Many didn't want to upgrade, so Google gave users a way to access the old Analytics.
But now, over a year later, Google has decided to completely stop supporting the old interface and require users to use the new one.
Those who have not upgraded are not happy because they are missing some familiar reports. But it is what it is and you got to upgrade.