What code have you used for this?
Is it just the code from the Wix Stores Backend API and the addProductMedia function itself?
https://www.wix.com/corvid/reference/wix-stores-backend.html#addProductMedia
As for the internal server error, is it this one as shown here?
https://support.wix.com/en/article/live-site-error-error-500-internal-server-error