Most likely you are running make install from the wrong directory. You must run it from the xblock-sdk directory.
The docker approach is probably easier - I tested these commands from the README on my computer now and confirmed they work:
git clone git@github.com:edx/xblock-sdk.git
cd xblock-sdk
docker build -t xblock-sdk .
docker run -d -p 8000:8000 --name xblock-sdk xblock-sdk
docker exec -it xblock-sdk python3 manage.py migrate
(Then it was running at http://localhost:8000/ )
Alternately, for the non-docker approach, the commands you need are as follows, but may not work if you don’t have the correct version of python or the correct system dependencies installed. However, I just tried these commands on my mac and they worked without issue.
git clone git@github.com:edx/xblock-sdk.git
cd xblock-sdk
python3 -m venv venv
source venv/bin/activate
make install
python3 manage.py migrate
python3 manage.py runserver
It’s probably worth mentioning that as an XBlock developer, I personally don’t use the XBlock SDK anymore, and just develop and test my XBlock directly in Studio+LMS. The XBlock SDK is useful for learning XBlocks and testing very simple XBlocks but for anything complex it acts differently than the LMS so is less useful.