Installing OSPD-Openvas

Hello,
I tried to install ospd-openvas, with the instruction given on: Building 21.04 from Source — Greenbone Community Documentation documentation
After many trys it always ends in the error: Could not install packages due to an EnvironmentError: 404 Client Error: Not Found for url: https://pypi.org/simple/v21-4-4/
Does someone know how to fix this error?