You should first retrieve the associated object and use that object as input in your log message.
To print the value from an association you need to first “Retrieve” it. When you log you can log the attributes from multiple entities, but you do have to specify them all in your “Log Message” template.
If you just want to inspect the values, you may find it easier to install a breakpoint in your microflow. When the breakpoint is reached you can look in the Variables tab and see the contents of all the current entities in scope.
Hope this helps.