I don't have apple tv app in appstore

Verify that you are eligible. If you have enjoyed an extended free trial before, then you can’t have another one. Redeem within the first 90 days on your new qualifying Apple device, in the TV app. Find the “Enjoy 3 Months Free” redeem button there.

Read here how to redeem: Apple TV+ offer - Apple

Read here about the conditions: Apple TV+ offer - Terms and conditions - Apple

Read here if you have trouble redeeming: If you can't redeem your promotion for an Apple subscription - Apple Support

↳ Step 5 says to contact Apple if the prior steps didn’t bring a solution. They can assist.

appletv+ is not an app it's a streaming service build into the build in appletv app in the appletv box

you can read about the appletv app here Apple TV app - Apple

and about appletv+ here Apple TV+ - Apple