This was support level 1's answer which didn't help: VMware Knowledge Base
I have been on the phone with support engineer (level 2?) for an hour and a half with him digging into our database, console, logs and such with nothing yet...
After waiting many days and updating iOS versions the apps all eventually installed on eight iPads having this error with no real answer . The support ticket was closed with no real answer.
I believe it works better at our school networks where there is not a global proxy requirement. The best answer for devices at home was to upgrade to the current iOS version 13.4.1 and older devices that cannot will still have 12064 error issues!
I have also just experienced this on a single app and on a single device..
In our case it was a teacher that had installed the app from Apple Appstore and we had forgot to add "Make App MDM Managed if User Installed" on the assignment!
We've had this happen for several users this week. We get "Install command failed" on an app, go to more-troubleshooting on the device and get the "Cannot retrieve license" error. When this happens the user can't install any applications and we see the same error message for every app. Then suddenly within a few days it seems to resolve itself and the apps start installing.
All VPP certs up to date, more than enough licenses, iOS up to date, enough storage, etc, etc.
Now i just fixed this by reuploading all my VPP certificates, even though they were well up to date, and syncing assets. Went to an iPad with this issue to test manual installation and it worked.
Not sure if this is a permanent solution as it sounds a bit too easy to be true, will have to keep an eye on it but it worked in this case.
EDIT: Seems it was a temporary solution, issue came back after a week or so.