When configuring a program line, it is only possible to assign dimension items from collections if the collection has been published for assignment.
For example, in the screenshot below, three collections have been published for the 'Branches' dimension, but only two have been made available for assignment. As such, it is only possible to select dimension items from these two collections.
An administrator can enable assignment on a dimension collection by navigating to Config > Structure > Dimensions, selecting a dimension, and clicking the 'View collections' button. Setting a collection to 'Assignment only' or 'Reporting and assignment' will allow the collection to be used for assignment when configuring a program line.