Using Outlook for Android in business environments for email and scheduling
I was having the same problem on Samsung galaxy s20.
The button is there but it's greyed out. So I was forced to use the built in browser.
I uninstalled the app and installed it through the samsung galaxy store (older version, so on other brands sideloading other version is worth a try). There the button isn't greyed out so I selected use default browser.
Afterwards I updated the app to the newest version and it was still selected en opening in default browser.
One problem solved, on to go, teslaunread keeps crashing on outlook notifications.