The first one is a dropdown list displaying a list of categories, and the second one is a data list that displays a list of users.
The data list options depend on the category selected in the dropdown list.
User selects category A, data list shows options 1-2-3,
User selects category B, data list shows options 4-5-6.
Here are the questions:
Should I disable the data list until a category has been selected in the dropdown list?
What should happen if the user changes the category after selecting an option in the data list?
Should the option in the data list be removed from the field?
If there are useful defaults to have in both lists, go for that option. Useful meaning that you have reason to believe that in at least 70% of cases these will be the options selected by the user. Alternatively - if that's the option that promotes your business goals the most, AND you're fine with preselecting it for the user (i.e. the cost of error is minimal and there are no ethical problems with that).
If not, disable the second list until the first one is selected and keep it in a "Please select..." state. Don't hide it because it might make the user wonder where that data is selected and might send them looking for it in other parts of the UI. You want your interface to be predictable.
If the user changes the category in the first dropdown, reset the second one to "Please select...".
Hide the dependent data list until a Category value is selected; this will avoid the user engaging with the data list until they should.
If the user changes the value in the Category dropdown, reset the dependent data list with updated values, clearing the user's previous selection. Don't show an error state - the user is simply selecting new information, no error has occurred.