No valid A or AAAA records could be ultimately resolved for www.iotplayground.tk. This means that Let's Encrypt would not be able to to connect to your domain to perform HTTP validation, since it would not know where to connect to.
No A or AAAA records found.
The current status as reported by the Let's Encrypt status page is Service Disruption as at 2021-06-29 14:02:56.605 +0000 UTC. Depending on the reported problem, this may affect certificate issuance. For more information, please visit the status page.
https://letsencrypt.status.io/