Tried many times to enable.. Still unable to enable. Even in developer options, under webview implementation android web view is not found. 

Why do you want to enable WebView? Chrome has WebView implementation now.

If you want to update to latest WebView, just update Chrome.

Some apps still using android web view only. Apps are not redirected to chrome for webview implementation. Apps are coded to use android webview app only. I am stuck in middle where i cannot force the app to use chrome web view implementation or to use android webview app. Online Payments are not able to do without that.

No, on Android 7.0 all applications that use WebView are actually getting an implementation of WebView loaded from the Chrome package, and no apps are using the disabled WebView package (they cannot even if they wanted to; the app does not get to choose this, it's decided by the OS). All you need to update is Chrome.

This is not the same thing as Chrome Custom Tabs, where apps load web pages in a cut-down version of the Chrome UI that they specifically choose to use.

The screenshots are attached.. Check them.
App Name : WALNUT
Payment Foreces the app to run Android web view package. When i click on that it is redirected to update app in playstore. When i press to enable it is not able to enable. Wheni click repeatedly to enable it automatically starts updating. After updating it immediately disabled.

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that is an error the app developer did. 

Please update Chrome, the error will go away. We will check if there is a way to reach the developer.

also please provide the full link to app playstore link.

Chrome is the only browser i trust and my daily driver. I have checked twice that my chrome is up to date. Chrome is upto date and i am attaching the screenshot. Really becoming more trouble for that program.

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Playstore app link : https://play.google.com/store/apps/details?id=com.daamitt.walnut.app

most likely the developer is trying to get the Webview Version and warn the user because of the issue described here:

https://www.chromium.org/developers/androidwebview/webview-ct-bug

However they are doing it wrong. We provided example code for developers if they want to do this:
https://github.com/ntfschr-chromium/ct_workaround

I will email the developers.

I have sent email to the app developer to look into this issue. Thank you for helping in this regard and I hope this will be solved as soon as possible. 

Finally issue was solved by looking into issue. New update was pushed and it was successful. Thanks for all developers who supported for this issue. Change the status to FIXED and close the thread.