Apple TV + is not an app - its a subscription service that can be accessed through the Apple TV app.
So, you'd need the Apple TV app, and it sounds like your Roku TV isnt a compatible model.
You never mentioned your Roku TV model # or firmware version: Settings/System/About
https://support.roku.com/article/360036652634
Is my Roku device compatible?
Not all Roku streaming devices support the Apple TV channel. To help determine if your device in your country is compatible, you need to know the device model. To find this information, go to the home screen on your Roku device and select Settings > System > About. The models below support the Apple TV channel.
Device Model
| Roku TV | 7000x, C000x, 8000x, A000x, 6000x |
| Roku Streambar™ | 9102 |
| Roku Streambar Pro | 9101R2 |
| Roku Smart Soundbar | 9101 |
| onn.™ • Roku Smart Soundbar | 9100 |
| Roku Express | 3900, 3930 |
| Roku Express+ | 3910, 3931 |
| Roku Express 4K | 3900, 3940 |
| Roku Express 4K+ | 3910, 3941 |
| Roku HD | 3932 |
| Roku Streaming Stick | 3600, 3800 |
| Roku Streaming Stick+ | 3810, 3811 |
| Roku Premiere | 3920, 4620 |
| Roku Premiere+ | 3921, 4630 |
| Roku Ultra | 4640, 4660, 4661, 4670, 4800 |
| Roku Ultra LT | 4662 |
| Roku 2 | 4205, 4210 |
| Roku 3 | 4200, 4230 |
