The post date on turnitin is the date and time you can start viewing feedback for your paper.
Has anyone not been able to view feedback past that date?

It has been 2 weeks since I submitted my essay, and the post date has passed. Is there something wrong with my paper, or does it sometimes just take them longer to mark essays than expected?

You may want to contact your professor or TA because it could just be delays in marking. If the grades haven't been posted on Avenue, this may be an indicator that the paper has yet to be marked.