The Turnitin API was depreciated (shut off). They sent notices about this over a year ago. If your school still has a contract, you need to use the LTI. If your school set up the LTI, you'll find Turnitin under "external tools". Here's a walkthrough:
How do I create a Turnitin assignment?
If you don't see it in the External Tools menu, contact your school's canvas admin to get the LTI installed.
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