First, check that the due date and time of your assignment has not passed. You will not be able to resubmit a paper after the assignment due date and time has passed. Please note that this is intended behavior. Resubmissions are not allowed once the assignment's due date and time has passed, even if the assignment allows for late submissions to be made. Late submissions are only available for first submissions.
If you are trying to resubmit before the due date/time and are still experiencing issues, it may be that resubmissions have not been enabled for the assignment. Assignments must have resubmissions allowed to enable you to resubmit your paper right up until the assignment's due date.
You will need to contact your instructor directly to check if resubmissions are allowed.
Please note that if you are using the New Standard Assignment (guidance on how to check which type of assignment you are using can be found here), and your instructor has allowed for resubmissions, you are only able to resubmit 3 times in a 24-hour period. If you have resubmitted more than 3 times within 24 hours, you will need to wait until the next calendar day to resubmit again.
If you are still unable to resubmit after checking the above, please reach out to the Turnitin Technical Support team.