There’s a known issue we’ve heard from a few customers and also seen documented by other MDM providers. Apple has also been notified, but no word on their resolution. In iOS 12.2 and higher, for MDM-managed devices using user-licensed VPP apps, end users will not be able to accept or cancel Terms and Conditions when accessing a VPP app for the first time. As such, app installations that require terms and conditions may fail, as they don’t appropriately get an accept or cancel response from the user on the device.
As a workaround, you can use device licensing when deploying VPP apps instead of a user license. Device licenses do not require terms and conditions. Alternatively, you can provide terms and conditions through a different channel than delivered with the app. You can switch from user-based apps to device-based apps through the guidance here: https://docs.microsoft.com/intune/vpp-apps-ios.  
Occasional Contributor

What is this "different channel"? I would love to know, I have been pulling my hair out for a while as my company just has a flurry of new hires start. I found the workaround to deploy to devices on my own last week, but would love to know how to have a user accept the VPP invitation without the AppStore App. Thanks in advance.

Frequent Visitor

Nice of you to acknowledge the issue after I was fighting it for a long time now :)

@Andrew Allston we are going to update the writeup. I was thinking we could get the terms across in a different mechanism, but in talking back with one of our Apple PM's, he said that can be tough so best approach would be to try the device-licensing as a workaround. Hope this helps.