Hi Therapong,
You can upgrade your Automic Workload Automation environment directly from version 21.0.4 to version 26.
System Compatibility
Your current infrastructure is perfectly positioned for this upgrade. Broadcom officially supports your stack for Version 26:
Operating System: Windows Server 2019 Datacenter is fully supported.
Database: SQL Server 2019 is fully supported.
What to Expect During the Upgrade
When upgrading an Automation Engine (AE) system directly from version 21.0.x to version 26.x, the process involves standard manual upgrade phases.
Check the below link for more information and let me know if you have further question on this .
https://docs.automic.com/documentation/webhelp/english/ALL/components/DOCU/latest/Automic%20Automation%20Guides/Content/Installation_Upgrade/install_upgrade_v210.htm
Regards
Vimal