Capistrano is using SSH to connect to the remote host.
Reading the error carefully it describes a problem making that connection.
First, verify you have SSH access to the host ‘manually’ (without running any scripts like Capistrano tasks).
If you are successful, from the same machine, with ssh root@api.medesko.com then Capistrano should not stumble.
This will require setting up the proper keys - for SSH communication - on your client and the host.
A simple Web search will yield numerous examples and descriptions how to do this. If your host is a provider like Digital Ocean they even offer tips and tools to make this very simple.
Most importantly: do not give up. Each time you struggle and find the solution to a problem you are LEARNING a new skill.