Where is the software S1TBX downloaded?

hello,I want to deal with the Sentinel -1A, where is the software S1TBX downloaded?

S1 Toolbox is a part of SNAP.
Select “Sentinel Toolboxes” or “All Toolboxes” here: http://step.esa.int/main/download/

BUT During the installation process, I found that Sentinel-1Toolbox could not be installed? Can’t handle S1A?

SNAP and S1 Toolbox are grey and disabled because they are needed by other toolboxes. If you uncheck the Radarsat-2 Toolbox you can disable the S1 Toolbox.
With the configuration you showed everything will be installed.