i just had this happen to me too on my iPad mini 1 with 9.3.5. A few days ago, I updated an app like always. Starting last night, I got the 'unable to process' error message. I tried restarting, changing the wifi network (something that has solved iTunes issues in the past), logging out and in, using Purchased instead of search results for the app or the Update list. If I am logged out, it doesn't even prompt me to log in if I click to update. I already called Apple, and the tech new nothing, which doesn't surprise me since Apple doesn't seem to tell them anything either when they discontinue, have bugs, or make changes like this. I don't even get the prompt to tell me to update to 9.6.3 anymore on the device or on iTunes.
I believe that Apple cut off some older ios's in the last couple of days, and since they never fixed the bug on iTunes 12.6.5 to download apps (windows might still be working), the older devices are out of luck now. What a shame.