I noticed that on my sandbox (Winter '19) the option to match licenses from production is missing. What can be the cause?
Salesforce disables this functionality when the sandbox and the production environment associated with it are on different releases. In this example, the sandbox is on Winter '19 but the production environment is on Summer '18.
Matching production licenses requires that your sandbox and production organizations are on the same Salesforce release. If your sandbox has been upgraded to the next release—for example, during sandbox preview—but your production organization hasn't, you can't match production licenses.