As of build 3.8.11 of the Windscribe iOS app, we only support minimum iOS 15.0 or above. We do our best to have as much support for a wide array of devices and iOS version but there are sometimes important security patches or feature additions in higher iOS versions that our app requires.
Ideally, you upgrade to iOS 15 since it's important to keep up with OS updates for security reasons. However, if you have a version of iOS lower than iOS 15 and can't/don't want to upgrade iOS, you can still use the older version of the Windscribe app. You will simply not be able to upgrade beyond the latest version of Windscribe available to your device.
If you are unsure about the specifics of your Apple device, have a look at this help article: https://support.apple.com/en-us/HT201685