Hello Hubaaa,
When you download an app, you need to double click the side button in order to confirm the download. If the button is not working, that is why it's prompting you to turn on the AssistiveTouch. You can use one of these ways to turn it on:
• Go to Settings > Accessibility > Touch > AssistiveTouch, then turn on AssistiveTouch
• Use "Hey Siri" to say, “Turn on AssistiveTouch”
• Go to Settings > Accessibility > Accessibility Shortcut, then turn on AssistiveTouch
Use AssistiveTouch on your iPhone
To get the side button repaired, you'll need to have the device serviced. You can find available service options for iPhone here: iPhone Repair & Service
Cheers!