I am.not able to use vpn in purchase without google play services/store etc.
It shows error billing , error settings etc, this is very frustration.
How to fix this annoying issue ?

Hello,
this app is officially distributed only through Google Play, and licenses can only be purchased through Google Play.
Without the Google Play Store app, you can only use the free features that do not require a license.

So there is no way to use purchased app without google services?
Then this app is not worth it, since google services is not so safe.

You may or may not like it, but it's the go-to app store for Android.
In any case, everyone is free to form their own opinion, and while I don't agree with your conclusion, I respect it.

You do know that google services / google play store etc are spy on their users.
I don't think it's worth to have vpn client pro since it must have google play. I have disabled google play services and the rest i have uninstalled.
No spy on my devices.