Follow the instructions on this article: How do I add a device to Firefox Sync?

Same result as the last 3 times i followed the instructions. Both devices offer a code for the other device, but neither provides a place to enter the code.

Have you set up a sync account on your laptop?

If a device is already set up then you can use "I don't have the device with me" and enter the account name and sync key manually.

The problem was not having the sync key. Finally a screen appeared, on the laptop, and not the phone, that I hadn't encountered before, at the bottom of which appeared, in very small typeface, a link which said, "I lost my <device>." This allowed me to generate a new sync key and enter it on both laptop and phone.

Thanks to all for their suggestions.