The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.
The license feature will attempt to connect to the Metalogix server at https://license.metalogix.com/license/licenseservice.asmx on port 443 so make sure that it is allowed through the environment's firewall. Once that is done, test the activation, but it may fail with errors when making the SSL/TSL connection or with certificate validation errors. These can occur if certificates are missing in the local computer's Trusted Root and Intermediate Certification Authorities store or in the SharePoint Trusted Relationships.
[Root CA]
Go Daddy Class 2 Certification Authority
Thumbprint: ?27 96 ba e6 3f 18 01 e2 77 26 1b a0 d7 77 70 02 8f 20 ee e4
Download link: https://certs.godaddy.com/repository/gd-class2-root.cer
[Intermediate CA]
Go Daddy Secure Certification Authority
Thumbprint: ?7c 46 56 c3 06 1f 7f 4c 0d 67 b3 19 a8 55 f6 0e bc 11 fc 44
Download link: https://certs.godaddy.com/repository/gd_intermediate.crt