LowDown

I had the same problem. Running High Sierra on a Mac Mini. On Tuesday I saw there was an update shown in the App Store for Enpass, Adguard & Safari. I decided to update Enpass & Adguard first. Clicked on update for both apps while in the "update" tab of App Store and waited while the rotating busy icon displayed in the upper left corner of window. Finally, I figured it was hung up so closed and reopened App Store. Enpass & Adguard were no longer shown in the "update" tab, just Safari. I updated Safari no problem then went to the individual app pages for Enpass & Adguard where both gave me the option to update but neither would actually update. Just the spinning ball and nothing ever happened. So I tried it on my MacBook Air which is running Mojave. Both updated no problem. The problem persisted through yesterday when I finally broke down and updated my Mini to Mojave which I had been planning to do for months. Both apps updated fine on Mojave. Not a real solution for you but I never was able to get it to update on High Sierra. It appears to be an App Store problem since two separate apps had the same problem.