Thanks Oli!
That worked 
For others like me, what I did exactly was:
cd /var/snap/nextcloud/current/
mv certs certs1
reboot
nextcloud.enable-https lets-encrypt -d
and after that said that I was ready
nextcloud.enable-https lets-encrypt
rm -r certs1
I used nextcloud.enable-https -h to work out that the info in the wiki doesn’t tell you to put in the lets-encrypt option.
Is this new? I think I only used nextcloud.enable-https -d before, but that doesn’t work (Although I could well have just not remembered putting in lets-encrypt)
How do I know when the snap has been updated?