Thanks for those screenshots, Tonina.
Can you show us what the actual output of the command is?
I'm sure it shouldn't be the version beginning 8e520afa... from the site because that is the demo version that MediaTemple generated to show what it would look like, and the actual signature value that comes out is meant to be different. (MediaTemple is asking you to do this specifically because they don't know what the signature string will be, because only you can create it using your account key, because you're the only one who possesses the account key.)
As an alternative, you could consider using https://zerossl.com/, which has a web interface and concept similar to gethttpsforfree, but automates the process a bit more so that you have fewer tasks to do outside of your web browser while requesting your certificate. You could also consider choosing a hosting provider with better built-in Let's Encrypt support: