When I run mitmweb, I get "OSError 98 Address already in use" error

I’m running Linux Mint 19.1 Cinnamon. I’d like to use mitmweb and found the official site on https://mitmproxy.org/. I downloaded the “binary package” by clicking the big button and it downloaded https://snapshots.mitmproxy.org/4.0.4/mitmproxy-4.0.4-linux.tar.gz . How do I install/use the mitmweb file inside? I already did
chmod +x mitmweb

but got this error

~/programs/mitmproxy-4.0.4-linux $ ./mitmweb
Error starting proxy server: OSError(98, ‘Address already in use’)

It’s all explained in the documentation (launch mitmproxy, install cert, launch mitmweb)