Turnitin will release feedback and marks to students automatically on the Feedback Release Date that was entered when the assignment was set-up:

In the above example this will be on the 21st March 2023 at 2pm.
However, there may be an occasion where you have marked all submissions and would like to release the feedback and marks earlier. It is not possible to only release feedback and marks to certain students, it is either all or none.
To release the feedback and marks earlier than the originally intended date and time, navigate to the Turnitin Assignment Inbox and select the settings cog:

From the Assignment settings screen, change the Feedback Release Date to the date and time you would like the feedback and mark to be released, e.g. 03rd March 2023 at 2pm:

If you navigate to your Turnitin Assignment Inbox via the Blackboard Gradebook, you may notice in the Markable Items list view, a Post Marks option on the same row as the Turnitin Assignment:

Selecting to Post Marks will release the marks given to each student for their assignment, however, if the Feedback Release Date has not been reached then students will not be able to view their feedback.
Therefore, please ensure if you wish to release the feedback and marks early for students, do this via the Turnitin Assignment Inbox settings and editing the Feedback Release Date rather than posting the marks from Blackboard’s Gradebook.