In MS-Access, you can select a special value for your combo boxes called:
"Find a record on my form based on the value I selected in my combo box"
In older versions of MS-Access, all you had to do is select the option and be done with it. However, the option would have no effect if the form was not properly assigned a source query.
In order to paliate to that problem, Microsoft decided to check the current form status and, if unlinked, hide the option altogether so you cannot make a mistake.
The problem is that for many people the train of thought was not automatically that way.