Step-By-Step: Creating a SQL Server Always On Availability Group

Hi Myloking_ : to install an already created Stand-Alone instance or any additional feature to an existing clustered instance is not possible; 
 
Microsoft SQL Server 2008, Microsoft SQL Server 2008 R2, and Microsoft SQL Server 2012 and all editions onwards do not support adding or removing features or components to or from an existing failover cluster instance. For example, you cannot add the Database Engine to an existing Analysis Services-only failover cluster, nor can you add Analysis Services to an existing Database Engine-only failover cluster. On the same note, removing features from a clustered instance is also not supported: 
 
https://support.microsoft.com/en-us/topic/you-cannot-add-or-remove-features-to-a-sql-server-2008-sql-server-2008-r2-or-sql-server-2012-failover-cluster-0859a667-4b71-1f01-baee-0ac25562bdbe 
 
The same goes when trying to install an Instance not set as a node:
https://learn.microsoft.com/en-us/sql/sql-server/failover-clusters/windows/upgrade-a-sql-server-failover-cluster-instance?view=sql-server-ver16 
 
The following scenarios are not supported:
 
- You cannot migrate from a stand-alone instance of SQL Server to a failover cluster.
- Add features to a failover cluster. For example, you cannot add the Database Engine to an existing Analysis Services-only failover cluster.
- You cannot downgrade a failover cluster node to a stand-alone instance.
 
The option to add or remove features is not supported either when you run the setup through SQL Server Installation Center or when you run the setup from a command prompt. So the only optionwould be to create a cluster from scratch. I hope it helps.