Thank you for the follow up!
Does the side button work fine when performing it’s other duties? I.e., locking the device, performing pre-setup shortcuts by triple-clicking it. If not, learn what to do if a button or switch isn't working.
If the side button works fine for performing the other tasks I mentioned, please ensure that your device is fully updated.
If your device is fully updated, try performing a Forced Restart.
If that doesn’t help, please see askbarnabas’ User Tip: Double-Click side button usage for Apple … - Apple Community
In addition, some users have found that trying different click speeds fixes this problem:
Go to: Settings > Accessibility > Side Button. Now, at the very top of the page, you will see: CLICK SPEED. Adjust this and then try to perform the function you’re having issues performing. See screenshot:

If you’re still having issues with using the side button for making purchases, please Contact Apple for support and service - Apple Support.
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If you’re trying to make online In-App Purchases, and you see: “Purchase could not be completed”, please review the very helpful reply by Raicya in this thread: your purchase could not be completed - Apple Community.
If that’s the case, your side button works just fine, and you have an issue with your payment method/s.
I hope this is helpful. Good Luck!