Smart TV app missing or no longer supported

App availability depends on the TV model, operating system, country and provider.

Never open TVs or power supplies. Stop using them if there is a burning smell, smoke or crackling. Do not disconnect power during an update.

Record the exact TV model and search its official app store for the app.
Check with the app provider whether that model is still supported. A working TV can be too old for a particular app.
Check for an official TV update in settings. Do not disconnect power during installation.
Do not install unknown app files or change region speculatively. If support has ended, a compatible external streaming device may be an alternative; check requirements before buying.

Test after each step and stop when the issue is solved. Checkmarks are not permanently saved.