You are on SQL Server 2014 SP3, and trying to upgrade Clarity to 15.5.1 or 15.6 ( which is supported as per the Release Notes)
Expected Results: To be able to upgrade successfully
Actual Results: Upgrade fails on Checkinstall stage with error similar to:
The database version could not be validated as a member of the supported versions. Current database info: Microsoft SQL Server 2014 (SP3-CU1) (KB4470220) - 12.0.6205.1 (X64) Nov 30 2018 02:59:03 Copyright (c) Microsoft Corporation Enterprise Edition (64-bit) on Windows NT 6.3 <X64> (Build 9600: ) (Hypervisor) . Check the checkinstall.log for connection errors. Refer to the Product Architecture Stack (PAS) for supported database versions.
This is DE48408, fixed in 15.6.1
Use the attached check-db-version.xml file where the script is corrected to accept MSSQL 2014 SP3.