That error means your computer can't connect to the Smart Setup server.
Just to be sure, we checked our servers here, and it's working fine.
So either it was temporary issue - and now you should be able to install now - or it's a problem with your computer, not being able to access our servers.
For example, you could try to connect to the following URL from your browser: https://api.landgraf.dev/tms/api/setup/user/products. You should get an "Unauthorized" response, which is fine, but are you getting such response, or is it timing out?