If you don't have premium access to an image asset, you should be blocked from downloading at the outset ("Upgrade to download") as shown in the screenshot below. Not run into PDF download errors/hangs that others are reporting here.
What I have found as the main issue for this not downloading is you don't have full premium access to the image or asset you are attempting to use in Adobe Express.
Pay for full image access. Also you can download them from Adobe stock images sit3e and upload the file you want in Express. This allowed me to convert my slides into a pdf document and I could download them. I'm shocked this hasn't been explained by support.
Apologies for the inconvenience. Can you share a screenshot of the template you used via private message? If not sure how to send through private message, click on this link. A screenshot of a few of the templates would help us identify the issue.