See my post here:
https://community.esri.com/t5/arcgis-enterprise-questions/11-2-upgrade-instant-apps-config-won-t-com...
We lost the Instant Apps configuration page and all but basic shapes in Map Viewer and Map Viewer Classic symbology picker when we upgraded from 11.1 to 11.2.
So we cannot create an Instant App or re-configure any of our existing Instant Apps. Nor can we modify anything but basic symbology in the map viewer.
I did receive 2 warnings during and after the post install. The first came during the post-install from our Carbon Black: "Threat Blocked". The second was a Post Install notice that said there was a problem with the language pack install. Since I could do nothing about either notice, I did nothing.
In the meantime:
I have been working with support for over a month and there is no solution found. For us, we did not lose basemaps from Living Atlas, but losing the other components (for us anyway, just as bad).
From the console logs, the Instant App config page appears to be looking for js and ts scripts as well as calcite components that for whatever reason did not install.
I say 'did not install' as opposed to 'did not install properly' or 'did not install in the right location' because my webgisdr backup files are now 2Gb smaller than my 11.1 webgisdr backups. We didn't delete 2Gb of items, layers etc so to me that suggests alot of libraries, etc didn't install.
Anyway, support asked me to search the portal gwdb for a user 'esri_en', items owned by user 'esri_en', and groups owned by user 'esri_en'. 'esri_en' as a user is not found, because 'esri_en' is either a language or a culture tag in an items info, but not a user. I do however recall seeing 'esri_en' in older versions of Living Atlas maps and layers.
At this point, they have now suggested to maybe try and Export Site and Import Site operations, based on information that those ops can sometimes repair corrupted installs. I'm not so sure. Plus there is not much help on this from the Portal rest api help.
I am more inclined to try a new install of 11.2 on a new server and perform a Join Site, as this is something we just did at 11.1
Wish I could help more. Our rep is going to try to escalate our case for us...