I was having the same issue, but I discovered what the problem is which is explained here:
https://help.figma.com/hc/en-us/articles/360040531773-Share-files-and-prototypes
So essentially you need to:
- Click the play button to open the prototype.
- Click the “Share Prototype” button.
- Click the “Copy link” button.
- You have a workable link to your prototype now.