Hi Cathy,
i had the same problem with one of my experiment. Turns out that for some reason on some versions of the experiment pavlovia fails to create a proper link for running the experiment. Try simply adding /html at the end of the link.
From: https://run.pavlovia.org/cathy-hauspie/rpep-study
To: https://run.pavlovia.org/cathy-hauspie/rpep-study/html
Worked for me!
tandy