You can automate the process using conditional activities.
Completion Tracking must be enabled at site level, and activated at course level.
Add your Feedback activity then in the settings scroll down to 'Activity completion'.
For 'Completion tracking' choose 'Show activity as complete when conditions are met' then tick the two boxes for 'Student must view this activity to complete it' and 'View as completed if the feedback is submitted'.
Return to the course and add a Turnitin assignment. In the 'Restrict access' settings choose the name of your feedback activity in the 'Activity completion condition' section, and choose 'must be marked complete'.
In the 'Before activity can be accessed' section choose 'Show activity greyed-out, with restriction information'
Now when you return to the course you'll see what the students will see.
The Turnitin assignment will be greyed-out and won't be unlocked until the Feedback activity has been submitted.