@louis Yes, that is the home directory of the instance user. I’ve tried installing it in a fresh CMK instance without issue, taking the steps as per the following screenshot:
Here are the commands I ran, so you may copy/paste them, if you need:
omd version
pwd
mkdir -pv local/lib/python3/cmk/gui/plugins/sidebar/
cd local/lib/python3/cmk/gui/plugins/sidebar/
curl -LO https://raw.githubusercontent.com/finalduty/checkmk-hostmatrix-snapin/main/host_matrix.py
ls -hal
md5sum host_matrix.py
omd restart
Please let me know if this helps, so I can update the documentation on Github as needed 