How to solve the problem if you see no users from the drop-down menu when you try to assign one when you create a new booking?
Most probably you have created a new project and just enabled the Resources module in the project settings.
Now you need to go to Administration (on top) > Users > [select the users you would like to be available as assignees on new bookings]. When you open such user, go to the Projects tab and add the project with the Resources plugin you are trying to create a new booking with. Afterward, you will get the selected users in the drop-down menu when you create a new booking.
Another possible problem that you might have in such scenario is to have blank issues list on the drop-down menu when creating a new booking. In this case, you have to go to Project settings > Issue tracking > [enable one or more trackers]. Now, go to Issues tab and create one or more issues. Then you will get issues in the drop-down menu of your new booking.
Video demonstration
To see the problem solving in action, please check the video below: