Some of our users reported that Core Shell/ Tunnel failed to complete their purchases and displayed a pop-up warning:
Reinstall
It's very likely a problem of the App Store or macOS, please try deleting the app and reinstalling it from the App Store again.
Don't worry, your data keeps untouched if you delete the app from Launchpad, all the profiles will be back after you reinstall Core Shell/ Tunnel.
WARN: do not uninstall the app using third-party tools such as "CleanMyMac".
Contact Apple for Support
If you still face this issue after did a reinstallation, the only solution we can suggest is do what Apple tells: contact iTunes Support.
We have no permission to handle the purchase process, Apple takes full control of this. Core Shell/ Tunnel simply receives what App Store responds: success or not.
This error happens more frequently when Apple improving there App Store or related infrastructures, you can check out Apple’s System Status Page, see the status of Mac App Store services:
Although we could do little on this issue, but if you need our assistance, don't hesitate to send me (@yang) a personal message.