The company that makes and maintains bootmapper client shut their doors in 2018. The software is no longer supported and can actually only be used under certain licenses, which you can no longer get because of said closure.
With that said QMK is a vey well developed firmware, with great community support and contributions. As of now, a GUI called VIA can be used to configure the keymaps simply. I believe this GUI is a suitable and much more robust replacement for Bootmapper Client. Give it a try when you get the chance!