In order to unassign a machine, you must make sure that you are at the site level where this machine is located. If, for example, you are on the customer center level, you will not have the option to unassign a machine.
In the example below, PC AIRPOWER is the wrong scope because the machine's site is "Mk5s Test Controllers". Therefore, when clicking on the three dots in the action-column, the unassign option does not appear. It just says EDIT.
To quickly switch to the right site, copy-paste the machine's serial number in the Search for Anything bar (top left, in the menu) and click on either of the results (under category machines or sites). Both results will change your scope to the site's level.
Now that the scope is correct, the unassign button is available.
Find more information about (re)assigning machines here.
