Technical Blog Post
Unable to add Domain to attribute in Database Configuration app
So, you are trying to change the domain on an attribute from a specific object, however the domain that you created in the Domains app is not in the select value list. You try to create a new domain, but it also doesn't show up.
What could be happening ?
A common cause for this kind of behavior is if for example you are trying to add an ALN data type domain to an attribute where the data type in not ALN (for example, the attribute type is set to UPPER instead).
Once you change the attribute data type to ALN, you should able to see the domain available in the list to select values, because both the Domain type and Attribute type will match this time.