First, double clicking side button (power button) does not open Apple Wallet. Double clicking side button activates Apple Pay, verifies your identity and prepares your default payment method for use.
Apple Wallet is an app and looks like this,

So, let’s get double clicking working again. Please follow this path,
iPhone > Settings > Accessibility > AssistiveTouch — disable, if it is disabled, momentarily enable and then disable again.
Scroll down to bottom of AssistiveTouch screen > Confirm with AssistiveTouch > disable
After confirming above, please do a forced restart following the instructions in the Apple Support article linked below. If you’re unfamiliar with the procedure, follow the step by step instructions in the article.
Force restart iPhone - Apple Support