Turnitin UK are currently reporting performance issues, submissions should still be possible.
Message below from Turnitin - for further updates please see https://turnitin.statuspage.io/
Investigating - We’ve run into some issues.
Due to an unexpected service incident, users of Turnitin and Turnitin UK may be experiencing issues with slowness when opening Similarity Reports, and receiving errors when attempting to grade or add feedback.
Users accessing these services via an integration may also be affected.
We invite you to track the progress of this incident while we work to bring the service back to optimum health. Jan 16, 2023 - 02:59 PST