Explanation
Two possible reasons that could cause the test org to not appear in the list of target organizations are:
The components in the Change set could not be found within the test org. This means that the test org does not have the same metadata components or features as the development org, and therefore cannot receive the change set.
The development org has not been approved to upload Change Sets from within the test org. This means that there is no deployment connection between the two orgs, and the test org has not authorized the development org to send change sets to it.
Verified References: https://help.salesforce.com/s/articleView?id=sf.changesets_about.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.changesets_create_outbound.htm&type=5