You cannot convert an existing standalone instance to an Failover Clustered Instance (FCI).
You can configure standalone instances to be part of an Availability Group, but FCIs use a separate install path, and must be configured from the point of install.
From the AWS docs
FCIs require some form of shared storage—disks on a storage area network (SAN), file shares on Server Message Blocks (SMBs), or locally attached storage with Storage Spaces Direct (S2D), SIOS Datakeeper, or Amazon FSx.
Amazon provides a very though overview and deployment guide for installing SQL Server FCIs using Amazon FSx. It is a comparatively complex configuration, compared to building standalone instances, or even Availability Groups.
Note that Amazon's recommendations in the deployment guide do not use AMIs with SQL Server already installed, but depend on your purchasing licensing on your own:
This Quick Start deploys MSSQL in Bring-Your-Own-License mode. It does not support deployment of MSSQL license-included Amazon Machine Images (AMIs).