We upgraded to 2.3.1 and get on product list pages of several stores an error:
No linked stock found
Go to Stores -> Inventory -> Stocks (admin/inventory/stock/edit/stock_id/1/) and enable it for all websites, then hit save.
I think this issue appears if you're using an older version of composer.
I had the same issue in my developer machine over and over again when testing our update from Magento 2.2.4 to 2.3.1 using composer 1.2.2 (which is the standard version in the Debian 9 Repo).
I also tested it in a Ubuntu 16.04 developer machine with composer 1.5.2 with no issues.
Then I removed composer version 1.2.2 from my debian 9 system and installed the latest version 1.8.6 and the issue disappeared.
Would be nice if someone could confirm my theory ;)
possible the issue also happens with data migration, using the magento provided data migration tool.
I have a clean installed 2.3.2 (using composer) and after a successful migration of all data (multi-store), I could not save due to same error.
The provided solution also fixed the problem, as only the default store was listed in the Default stock setup.