I'm having the same problem all of a sudden today. Some apps update fine, but so far all the google apps (google maps, chrome, google, sheets, youtube, gmail, google photos) and some banking apps (and weirdly enough a pdf scanner app) give me the exact same popup. When I click buy (even though I clearly own it...and it's free) the popup loops itself and doesn't go away unless I click cancel (and the updating stops). I've already checked out the other post and it's not a user-error problem. As in, my location is correct, my iOS is up to date, my account is correct, I've logged out and logged in again, restarted, nothing's wrong with my memory/storage. The only thing that's worked it deleting each app one by one and redownloading them. And since this is a problem with ALL my google apps (and a few select others) it's very tedious and annoying.
I hope there's an easier solution for others, because I can imagine this being a huge hassle for apps that maybe don't have a login or something to save your data and stuff and is only a local app. At least with the google suite I can just sign in and expect no changes to the data on the app.