Run the Sitecore Xdb Collection Update tool
In Sitecore XP 9.3.0, the Collection database schema was changed to store a shard key value. This is required by the Microsoft Split-Merge service that rebalances data between shards.
The Split-Merge functionality is only available if you are running Sitecore XP on Azure (not on-Prem), and the AlwaysEncrypted feature is not configured. It is only relevant if you want to split the data in databases to improve scaling.
The Sitecore Xdb Collection Update tool fills in the shard key values for data that has been collected before you upgraded. You can use it to update production databases while only placing a minimal load on the database server.
The tool is stored in the XConnect Upgrade Tool.zip package that is part of the Sitecore 10.X.0 rev. XXXXXX (upgrade files).zip package that you downloaded earlier.
You must run the Sitecore Xdb Collection Update tool before you can use the Microsoft Split-Merge service.
For more information about how to configure Sitecore XP to use the Microsoft Split-Merge service, see the Sitecore documentation.