In certain environments, installing the Teams app might not be possible, not even by the team owner.
There are three ways to enable the app anyway, though an approval of a Microsoft Teams administrator will be required.
Option 1: Allow team owners to install apps by themselves
MICROSOFT TEAMS ADMIN MICROSOFT 365 ADMIN
This is the option Microsoft recommends. This option is active by default.
Please note: It may take up to 24 hours for the app to be visible for users in Teams.
Option 2: Install the app for certain teams manually
MICROSOFT TEAMS ADMIN MICROSOFT 365 ADMIN
If non of the above works, Microsoft Teams administrators can also add apps on an individual basis.
In case the app is not allowed by default, it might also be necessary to allow the app first (see above).
Option 3: Choose the app to be installed for all/certain users by default
Heads up
In case you see this error below, it usually works and the app should be installed. There currently seems to be a bug on the Microsoft side regarding this.