I think, you are looking for this tool: https://gpuopen.com/gaming-product/compressonator/
Since version 2.3, AMD/ATI Compress was renamed to Compressonator and moved to GPUOpen [https://gpuopen.com/compressonator-is-going-open-source/ ].
Older SDKs are no longer available on developer central. Please use the Compressonator SDKs available here:
For any query or issue regarding Compressonator, please post here: Issues · GPUOpen-Tools/Compressonator · GitHub