On Linux operating systems, after an upgrade of Portal for ArcGIS 10.5.1 to 10.6 with Insights for ArcGIS 2.1 installed, the Portal Upgrade page returns the following error message:
Error: There was an error when attempting to upgrade the existing portal. Failed to start the database server.
The exact cause of this error is currently under investigation. This article will be updated with further information when it becomes available.
The Portal Service must be restarted for the upgrade to continue normally.