Hello @SineadSheehan - I also use Canvas and Turnitin as an instructor and would like to help. Our University has Canvas along with the Canvas Plagiarism Framework set up with Turnitin and instructors access the files and similarity reports via Canvas Grades SpeedGrader. I mention this as I understand there are different ways to configure and access Turnitin. For example, Turnitin also has a grading tool for assignments via their website at turnitin.com. If you are accustomed to using the turnitin.com site for grading and a feature set (e.g., GradeMark, Feedback Studio, and Gradescope) you have been using is longer available, then I presume your institution either discontinued the subscription for that feature or perhaps the system configuration changed, perhaps in error. Either way, I would reach out to your Canvas Admin and/or IT Dept. to make an inquiry. In addition, the Turnitin guides may be useful: https://guides.turnitin.com/hc/en-us
But here is how I access the student submissions from within Canvas: from Grades, I access SpeedGrader via the three dots to the right side of the assignment name. I can also go to the assignment page and click the SpeedGrader button there.
SpeedGrader Link