An Unknown Host error like the following occurs when you use the wrong DNS hostname or there are underlying problems with the DNS resolution:
ERROR 2005 (HY000): Unknown MySQL server host 'myqsl--do-user-example-0.db.ondigitalocean.com' (0)
If you are getting this error, first verify that you are using the correct hostname to connect by following How to Connect to MySQL Database Clusters. Then, verify whether the machine you’re connecting from has problems with DNS resolution by following Why does my domain fail to resolve?