Double tap to download an App not working

Change side button press speed -

Many have found that slowing down the press speed alleviates this problem.

Go to Settings>Accessibility>Side Button. Select Slow for the button.

M1 Mac mini, macOS 14.5;  Watch, series 7 GPS/cellular, watchOS 10.5; iPhone 12 Pro, iOS 17.5; TV 4th Gen, tvOS 17.5; HomePods, audioOS 17.5 and Airport Express w/ Airplay 2

When a “Double Click to Install/ Confirm/ Pay” pop-up appears, simply press the Side button (the physical button on the right side) twice quickly. Upon confirming, the iPhone will do the final authentication using Face ID and let you install the particular app.