After filling in all the information it is necessary to associate a role to the user.
Note that you can only activate a user after at least one role is assigned to him.
At the bottom of the User's screen, on the tab "Roles" click on "Link" and choose a role (e.g. requester profile, executor profile, ...)
In case you want to remove a Role, you should select the profile and click on the option "Unlink".